Chapter XXIII: The Platinum Metals (2)

The primary source from which the compounds of ruthenium and osmium
are obtained is either _osmiridium_ (the osmium predominates, from
IrOs to IrOs_{4}, sp. gr. from 16 to 21), which occurs in platinum
ores (it is distinguished from the grains of platinum by its
crystalline structure, hardness, and insolubility in aqua regia),
or else those insoluble residues which are obtained, as we saw
above, after treating platinum with aqua regia. Osmium predominates
in these materials, which sometimes contain from 30 p.c. to 40 p.c.
of it, and rarely more than 4 p.c. to 5 p.c. of ruthenium. The
process for their treatment is as follows: they are first fused
with 6 parts of zinc, and the zinc is then extracted with dilute
hydrochloric acid. The osmiridium thus treated is, according to
Fritzsche and Struvé's method, then added to a fused mixture of
potassium hydroxide and chlorate in an iron crucible; the mass as
it begins to evolve oxygen acts on the metal, and the reaction
afterwards proceeds spontaneously. The dark product is treated with
water, and gives a solution of osmium and ruthenium in the form of
soluble salts, R_{2}OsO_{4} and R_{2}RuO_{4}, whilst the insoluble
residue contains a mixture of oxides of iridium (and some osmium,
rhodium, and ruthenium), and grains of metallic iridium still
unacted on. According to Frémy's method the lumps of osmiridium are
straightway heated to whiteness in a porcelain tube in a stream of
air or oxygen, when the very volatile osmic anhydride is obtained
directly, and is collected in a well-cooled receiver, whilst the
ruthenium gives a crystalline sublimate of the dioxide, RuO_{2},
which is, however, very difficultly volatile (it volatilises
together with osmic anhydride), and therefore remains in the cooler
portions of the tube; this method does not give volatile ruthenic
anhydride, and the iridium and other metals are not oxidised or
give non-volatile products. This method is simple, and at once
gives dry, pure osmic anhydride in the receiver, and ruthenium
dioxide in the sublimate. The air which passes through the tube
should be previously passed through sulphuric acid, not only in
order to dry it, but also to remove the organic and reducing dust.
The vapour of osmic anhydride must be powerfully cooled, and
ultimately passed over caustic potash. A third mode of treatment,
which is most frequently employed, was proposed by Wöhler, and
consists in slightly heating (in order that the sodium chloride
should not melt) an intimate mixture of osmiridium and common salt
in a stream of moist chlorine. The metals then form compounds with
chlorine and sodium chloride, whilst the osmium forms the chloride,
OsCl_{4}, which reacts with the moisture, and gives osmic
anhydride, which is condensed. The ruthenium in this, as in the
other processes, does not directly give ruthenic anhydride, but is
always extracted as the soluble ruthenium salt, K_{2}RuO_{4},
obtained by fusion with potassium hydroxide and chlorate or
nitrate. When the orange-coloured ruthenate, K_{2}RuO_{4}, is mixed
with acids, the liberated ruthenic acid immediately decomposes into
the volatile ruthenic anhydride and the insoluble ruthenic oxide:
2K_{2}RuO_{4} + 4HNO_{3} = RuO_{4} + RuO_{2},2H_{2}O + 4KNO_{3}.
When once one of the above compounds of ruthenium or osmium is
procured it is easy to obtain all the remaining compounds, and by
reduction (by metals, hydrogen, formic acid, &c.) the metals
themselves.

Osmic anhydride, OsO_{4}, is very easily deoxidised by many
methods. It blackens organic substances, owing to reduction, and is
therefore used in investigating vegetable and animal, and
especially nerve, preparations under the microscope. Although osmic
anhydride may be distilled in hydrogen, still complete reduction is
accomplished when a mixture of hydrogen and osmic anhydride is
slightly ignited (just before it inflames). If osmium be placed in
the flame it is oxidised, and gives vapours of osmic anhydride,
which become reduced, and the flame gives a brilliant light. Osmic
anhydride deflagrates like nitre on red-hot charcoal; zinc, and
even mercury and silver, reduce osmic anhydride from its aqueous
solutions into the lower oxides or metal; such reducing agents as
hydrogen sulphide, ferrous sulphate, or sulphurous anhydride,
alcohol, &c., act in the same manner with great ease.

The lower oxides of osmium, ruthenium, and of the other elements of
the platinum series are not volatile, and it is noteworthy that the
other elements behave differently. On comparing SO_{2}, SO_{3};
As_{2}O_{3}, As_{2}O_{5}; P_{2}O_{3}, P_{2}O_{5}; CO, CO_{2}, &c.,
we observe a converse phenomenon; the higher oxides are less
volatile than the lower. In the case of osmium all the oxides, with
the exception of the highest, are non-volatile, and it may
therefore be thought that this higher form is more simply
constituted than the lower. It is possible that osmic oxide,
OsO_{2}, stands in the same relation to the anhydride as C_{2}H_{4}
to CH_{4}--_i.e._ the lower oxide is perhaps Os_{2}O_{4}, or is
still more polymerised, which would explain why the lower oxides,
having a greater molecular weight, are less volatile than the
higher oxides, just as we saw in the case of the nitrogen oxides,
N_{2}O and NO.

_Ruthenium and osmium_, obtained by the ignition or reduction of
their compounds in the form of powder, have a density considerably
less than in the fused form, and differ in this condition in their
capacity for reaction; they are much more difficultly fused than
platinum and iridium, although ruthenium is more fusible than
osmium. Ruthenium in powder has a specific gravity of 8·5, the
fused metal of 12·2; osmium in powder has a specific gravity of
20·0, and when semi-fused--or, more strictly speaking,
agglomerated--in the oxyhydrogen flame, of 21·4, and fused 22·5.
The powder of slightly-heated osmium oxidises very easily in the
air, and when ignited burns like tinder, directly forming the
odoriferous osmic anhydride (hence its name, from the Greek word
signifying odour); ruthenium also oxidises when heated in air, but
with more difficulty, forming the oxide RuO_{2}. The oxides of the
types RO, R_{2}O_{3}, and RO_{2} (and their hydrates) obtained by
reduction from the higher oxides, and also from the chlorides, are
analogous to those given by the other platinum metals, in which
respect osmium and ruthenium closely resemble them. We may also
remark that ruthenium has been found in the platinum deposits of
Borneo in the form of _laurite_, Ru_{2}S_{3}, in grey octahedra of
sp. gr. 7·0.

For osmium, Moraht and Wischin (1893) obtained free osmic acid,
H_{2}OsO_{4}, by decomposing K_{2}OsO_{4} with water, and
precipitating with alcohol in a current of hydrogen (because in air
volatile OsO_{4} is formed); with H_{2}S, osmic acid gives
OsO_{3}(HS)_{2} at the ordinary temperature.

Debray and Joly showed that ruthenic anhydride, RuO_{4}, fuses at
25°, boils at 100°, and evolves oxygen when dissolved in potash,
forming the salt KRuO_{4} (not isomorphous with potassium
permanganate).

Joly (1891), who studied the ruthenium compounds in greater detail,
showed that the easily-formed KRuO_{4} gives RuKO_{4}RuO_{3} when
ignited, but it resembles KMnO_{4} in many respects. In general, Ru
has much in common with Mn. Joly (1889) also showed that if KNO_{3}
be added to a solution of RuCl_{3} containing HCl, the solution
becomes hot, and a salt, RuCl_{3}NO_{2}KCl, is formed, which enters
into double decomposition and is very stable. Moreover, if RuCl_{3}
be treated with an excess of nitric acid, it forms a salt,
RuCl_{3}NOH_{2}O, after being heated (to boiling) and the addition
of HCl. The vapour density of RuO_{4}, determined by Debray and
Joly, corresponds to that formula.

[10] Although palladium gives the same types of combination (with
chlorine) as platinum, its reduction to RX_{2} is incomparably
easier than that of platinic chloride, and in the case of iridium
it is also very easy. Iridic chloride, IrCl_{4}, acts as an
oxidising agent, readily parts with a fourth of its chlorine to a
number of substances, readily evolves chlorine when heated, and it
is only at low temperatures that chlorine and aqua regia convert
iridium into iridic chloride. In disengaging chlorine iridium more
often and easily gives the very stable iridious chloride, IrCl_{3}
(perhaps this substance is Ir_{2}Cl_{6} = IrCl_{2},IrCl_{4},
insoluble in water, but soluble in potassium chloride, because it
forms the double salt K_{3}IrCl_{6}), than the dichloride, IrCl_2.
This compound, corresponding to IrX_{2}, is very stable, and
corresponds with the _basic oxide_, Ir_{2}O_{3}, resembling the
oxides Fe_{2}O_{3}, Co_{2}O_{3}. To this form there correspond
ammoniacal compounds similar to those given by cobaltic oxide.
Although iridium also gives an acid in the form of the salt
K_{2}Ir_{2}O_{7}, it does not, like iron (and chromium), form the
corresponding chloride, IrCl_{6}. In general, in this as in the
other elements, it is impossible to predict the chlorine compounds
from those of oxygen. Just as there is no chloride SCl_{6}, but
only SCl_{2}, so also, although IrO_{3} exists, IrCl_{6} is
wanting, the only chloride being IrCl_{4}, and this is unstable,
like SCl_{2}, and easily parts with its chlorine. In this respect
rhodium is very much like iridium (as platinum is like palladium).
For RhCl_{4} decomposes with extreme ease, whilst rhodium
chloride, RhCl_{3}, is very stable, like many of the salts of the
type RhX_{3}, although like the platinum elements these salts are
easily reduced to metal by the action of heat and powerful
reagents. There is as close a resemblance between osmium and
ruthenium. Osmium when submitted to the action of dry chlorine
gives osmic chloride, OsCl_{4}, but the latter is converted by
water (as is osmium by moist chlorine) into osmic anhydride,
although the greater portion is then decomposed into Os(HO)_{4}
and 4HCl, like a chloranhydride of an acid. In general this acid
character is more developed in osmium than in platinum and
iridium. Having parted with chlorine, osmic chloride, OsCl_{4},
gives the unstable trichloride, OsCl_{3}, and the stable soluble
dichloride, OsCl_{2}, which corresponds with platinous chloride in
its properties and reactions. The relation of ruthenium to the
halogens is of the same nature.

Platinum and its analogues, like iron and its analogues, are able to form complex and comparatively stable cyanogen and ammonia compounds, corresponding with the ferrocyanides and the ammoniacal compounds of cobalt, which we have already considered in the preceding chapter.

If platinous chloride, PtCl_{2} (insoluble in water), be added by degrees to a solution of potassium cyanide, it is completely dissolved (like silver chloride), and on evaporating the solution deposits rhombic prisms of _potassium platinocyanide_, PtK_{2}(CN)_{4},3H_{2}O. This salt, like all those corresponding with it, has a remarkable play of colours, due to the phenomena of dichromism, and even polychromism, natural to all the platinocyanides. Thus it is yellow and reflects a bright blue light. It is easily soluble in water, effloresces in air, then turns red, and at 100° orange, when it loses all its water. The loss of water does not destroy its stability--that is, it still remains unchanged, and its stability is further shown by the fact that it is formed when potassium ferrocyanide, K_{4}Fe(CN)_{6}, is heated with platinum black. This salt, first obtained by Gmelin, shows a neutral reaction with litmus; it is exceedingly stable under the action of air, like potassium ferrocyanide, which it resembles in many respects. Thus the platinum in it cannot be detected by reagents such as sulphuretted hydrogen; the potassium may be replaced by other metals by the action of their salts, so that it corresponds with a whole series of compounds, R_{2}Pt(CN)_{4}, and it is stable, although the potassium cyanide and platinous salts, of which it is composed, individually easily undergo change. When treated with oxidising agents it passes, like the ferrocyanide, into a higher form of combination of platinum. If salts of silver be added to its solution, it gives a heavy white precipitate of silver platinocyanide, PtAg_{2}(CN)_{4}, which when suspended in water and treated with sulphuretted hydrogen, enters into double decomposition with the latter and forms insoluble silver sulphide, Ag_{2}S, and soluble _hydroplatinocyanic acid_, H_{2}Pt(CN)_{4}. If potassium platinocyanide be mixed with an equivalent quantity of sulphuric acid, the hydroplatinocyanic acid liberated may be extracted by a mixture of alcohol and ether. The ethereal solution, when evaporated in a desiccator, deposits bright red crystals of the composition PtH_{2}(CN)_{4},5H_{2}O. This acid colours litmus paper, liberates carbonic anhydride from sodium carbonate, and saturates alkalis, so that it presents an analogy to hydroferrocyanic acid.[11]

[11] This acid character is explained by the influence of the platinum
on the hydrogen, and by the attachment of the cyanogen groups.
Thus cyanuric acid, H_{3}(CN)_{3}O_{3}, is an energetic acid
compared with cyanic acid, HCNO. And the formation of a compound
with five molecules of water of crystallisation,
(PtH_{2}(CN)_{4},5H_{2}O), confirms the opinion that platinum is
able to form compounds of still higher types than that expressed
in its saline compounds, and, moreover, the combination of
hydroplatinocyanic acid with water does not reach the limit of the
compounds which appears in PtCl_{4},2HCl,6H_{2}O.

A whole series of _platinocyanides_ of the common type
PtR_{2}(CN)_{4}_n_H_{2}O is obtained by means of double
decomposition with the potassium or hydrogen or silver salts. For
example, the salts of sodium and lithium contain, like the
potassium salt, three molecules of water. The sodium salt is
soluble in water and alcohol. The ammonium salt has the
composition Pt(NH_{4})_{2}(CN)_{4},2H_{2}O and gives crystals
which reflect blue and rose-coloured light. This ammonium salt
decomposes at 300°, with evolution of water and ammonium cyanide,
leaving a greenish _platinum dicyanide_, Pt(CN)_{2}, which is
insoluble in water and acid but dissolves in potassium cyanide,
hydrocyanic acid, and other cyanides. The same platinous cyanide
is obtained by the action of sulphuric acid on the potassium salts
in the form of a reddish-brown amorphous precipitate. The most
characteristic of the platinocyanides are those of the alkaline
earths. The magnesium salt PtMg(CN)_{4},7H_{2}O crystallises in
regular prisms, whose side faces are of a metallic green colour
and terminal planes dark blue. It shows a carmine-red colour along
the main axis, and dark red along the lateral axes; it easily
loses water, (2H_{2}O), at 40°, and then turns blue (it then
contains 5H_{2}O, which is frequently the case with the
platinocyanides). Its aqueous solution is colourless, and an
alcoholic solution deposits yellow crystals. The remainder of the
water is given off at 230°. It is obtained by saturating
platinocyanic acid with magnesia, or else by double decomposition
between the barium salt and magnesium sulphate. The strontium
salt, SrPt(CN)_{4},4H_{2}O crystallises in milk-white plates
having a violet and green iridescence. When it effloresces in a
desiccator, its surfaces have a violet and metallic green
iridescence. A colourless solution of the barium salt
PtBa(CN)_{4},4H_{2}O is obtained by saturating a solution of
hydroplatinocyanic acid with baryta, or by boiling the insoluble
copper platinocyanide in baryta water. It crystallises in
monoclinic prisms of a yellow colour, with blue and green
iridescence; it loses half its water at 100°, and the whole at
150°. The ethyl salt, Pt(C_{2}H_{5})_{2}(CN)_{4},2H_{2}0, is also
very characteristic; its crystals are isomorphous with those of
the potassium salt, and are obtained by passing hydrochloric acid
into an alcoholic solution of hydroplatinocyanic acid. The
facility with which they crystallise, the regularity of their
forms, and their remarkable play of colours, renders the
preparation of the platinocyanides one of the most attractive
lessons of the laboratory.

By the action of chlorine or dilute nitric acid, the
platinocyanides are converted into salts of the composition
PtM_{2}(CN)_{5}, which corresponds with Pt(CN)_{3},2KCN--that is,
they express the type of a non-existent form of oxidation of
platinum, PtX_{3} (_i.e._ oxide Pt_{2}O_{3}), just as potassium
ferricyanide (FeCy_{3},3KCy) corresponds with ferric oxide, and
the ferrocyanide corresponds with the ferrous oxide. The potassium
salt of this series contains PtK_{2}(CN)_{5},3H_{2}O, and forms
brown regular prisms with a metallic lustre, and is soluble in
water but insoluble in alcohol. Alkalis re-convert this compound
into the ordinary platinocyanide K_{2}Pt(CN)_{4}, taking up the
excess of cyanogen. It is remarkable that the salts of the type
PtM_{2}Cy_{5} contain the same amount of water of crystallisation
as those of the type PtM_{2}Cy_{4}. Thus the salts of potassium
and lithium contain three, and the salt of magnesium seven,
molecules of water, like the corresponding salts of the type of
platinous oxide. Moreover, neither platinum nor any of its
associates gives any cyanogen compound corresponding with the
oxide, _i.e._ having the composition PtK_{2}Cy_{6}, just as there
are no compounds higher than those which correspond to
RCy_{3}_n_MCy_{3} for cobalt or iron. This would appear to
indicate the absence of any such cyanides, and indeed, for no
element are there yet known any poly-cyanides containing more than
three equivalents of cyanogen for one equivalent of the element.
The phenomenon is perhaps connected with the faculty of cyanogen
of giving tricyanogen polymerides, such as cyanuric acid, solid
cyanogen chloride, &c. Under the action of an excess of chlorine,
a solution of PtK_{2}(CN)_{4} gives (besides PtK_{2}Cy_{5}) a
product PtK_{2}Cy_{4}Cl_{2}, which evidently contains the form
PtX_{4}, but at first the action of the chlorine (or the
electrolysis of, or addition of dilute peroxide of hydrogen to, a
solution of PtK_{2}Cy_{4}, acidulated with hydrochloric acid)
produces an easily soluble intermediate salt which crystallises in
thin copper-red needles (Wilm, Hadow, 1889). It only contains a
small amount of chlorine, and apparently corresponds to a compound
5PtK_{2}Cy_{4} + PtK_{2}Cy_{4}Cl_{2} + 24H_{2}O. Under the action
of an excess of ammonia both these chlorine products are converted
either completely or in part (according to Wilm ammonia does not
act upon PtK_{2}Cy_{4}) into PtCy_{2},2NH_{3}, _i.e._ a
platino-ammonia compound (_see_ further on). It is also necessary
to pay attention to the fact that ruthenium and osmium--which, as
we know, give higher forms of oxidation than platinum--are also
able to combine with a larger proportion of potassium cyanide (but
not of cyanogen) than platinum. Thus ruthenium forms a crystalline
_hydroruthenocyanic acid_, RuH_{4}(CN)_{6}, which is soluble in
water and alcohol, and corresponds with the salts M_{4}Ru(CN)_{6}.
There are exactly similar osmic compounds--for example,
K_{4}Os(CN)_{6},3H_{2}O. The latter is obtained in the form of
colourless, sparingly-soluble regular tablets on evaporating the
solution obtained from a fused mixture of potassium osmiochloride,
K_{2}OsCl_{6}, and potassium cyanide. These osmic and ruthenic
compounds fully correspond with potassium ferrocyanide,
K_{4}Fe(CN)_{6},3H_{2}O, not only in their composition but also in
their crystalline form and reactions, which again demonstrates the
close analogy between iron, ruthenium, and osmium, which we have
shown by giving these three elements a similar position (in the
eighth group) in the periodic system. For rhodium and iridium only
salts of the same type as the ferricyanides, M_{3}RCy_{6}, are
known, and for palladium only of the type M_{2}PdCy_{4}, which are
analogous to the platinum salts. In all these examples a
_constancy of the types_ of the double cyanides is apparent. In
the eighth group we have iron, cobalt, nickel, copper, and their
analogues ruthenium, rhodium, palladium, silver, and also osmium,
iridium, platinum, gold. The double cyanides of iron, ruthenium,
osmium have the type K_{4}R(CN)_{6}; of cobalt, rhodium, iridium,
the type K_{3}R(CN)_{6}; of nickel, palladium, platinum the type
K_{2}R(CN)_{4} and K_{2}R(CN)_{5}; and for copper, silver, gold
there are known KR(CN)_{2}, so that the presence of 4, 3, 2, and 1
atoms of potassium corresponds with the order of the elements in
the periodic system. Those types which we have seen in the
ferrocyanides and ferricyanides of iron repeat themselves in all
the platinoid metals, and this naturally leads to the conclusion
that the formation of similar so-called double salts is of exactly
the same nature as that of the ordinary salts. If, in expressing
the union of the elements in the oxygen salts, the existence of an
_aqueous residue_ (hydroxyl group) be admitted, in which the
hydrogen is replaced by a metal, we have then only to apply this
mode of expression to the double salts and the analogy will be
obvious, if only we remember that Cl_{2}, (CN)_{2}, SO_{4}, &c.,
are equivalent to O, as we see in RO, RCl_{2}, RSO_{4}, &c. They
all = X_{2}, and, therefore, in point of fact, wherever X (= Cl or
OH, &c.) can be placed, there (Cl_{2}H), (SO_{4}H), &c., can also
stand. And as Cl_{2}H = Cl + HCl and SO_{4}H = OH + SO_{3}, &c.,
it follows that molecules HCl or SO_{3}, or, in general, whole
molecules--for instance, NH_{3}, H_{2}O, salts, &c., can annex
themselves to a compound containing X. (This is an indirect
consequence of the law of substitution which explains the origin
of double salts, ammonia compounds, compounds with water of
crystallisation, &c., by one general method.) Thus the double salt
MgSO_{4},K_{2}SO_{4}, according to this reasoning, _may be_
considered as a substance of the same type as MgCl_{2}, namely, as
= Mg(SO_{4}K)_{2}, and the alums as derived from Al(OH)(SO_{4}),
namely, as Al(SO_{4}K)(SO_{4}). Without stopping to pursue this
digression further, we will apply these considerations to the type
of the ferrocyanides and ferricyanides and their platinum
analogues. Such a salt as K_{2}PtCy_{4} may accordingly be
regarded as Pt(Cy_{2}K)_{2}, like Pt(OH)_{2}; and such a salt as
PtK_{2}Cy_{5} as PtCy(Cy_{2}K)_{2}, the analogue of PtX(OH)_{2},
or AlX(OH)_{2}, and other compounds of the type RX_{3}. Potassium
ferricyanide and the analogous compounds of cobalt, iridium, and
rhodium, belong to the same type, with the same difference as
there is between RX(OH)_{2} and R(OH)_{3}, since FeK_{3}Cy_{6} =
Fe(Cy_{2}K)_{3}. Limiting myself to these considerations, which
may partially elucidate the nature of double salts, I will now
pass again to the complex saline compounds known for platinum.

(_A_) On mixing a solution of potassium thiocyanate with a
solution of potassium platinosochloride, K_{2}PtCl_{4}, they form
a double thiocyanate, PtK_{2}(CNS)_{4}, which is easily soluble in
water and alcohol, crystallises in red prisms, and gives an
orange-coloured solution, which precipitates salts of the heavy
metals. The action of sulphuric acid on the lead salt of the same
type gives the acid itself, PtH_{2}(SCN)_{4}, which corresponds
with these salts. The type of these compounds is evidently the
same as that of the cyanides.

(_B_) _Platinous chloride_, PtCl_{2}, which is insoluble in water,
forms _double salts with the metallic chlorides_. These double
chlorides are soluble in water, and capable of crystallising.
Hence when a hydrochloric acid solution of platinous chloride is
mixed with solutions of metallic salts and evaporated it forms
crystalline salts of a red or yellow colour. Thus, for example,
the potassium salt, PtK_{2}Cl_{4}, is red, and easily soluble in
water; the sodium salt is also soluble in alcohol; the barium
salt, PtBaCl_{4},3H_{2}O, is soluble in water, but the silver
salt, PtAg_{2}Cl_{4}, is insoluble in water, and may be used for
obtaining the remaining salts by means of double decomposition
with their chlorides.

(_C_) A remarkable example of the complex compounds of platinum
was observed by Schützenberger (1868). He showed that
finely-divided platinum in the presence of chlorine and carbonic
oxide at 250°-300° gives phosgene and a volatile compound
containing platinum. The same substance is formed by the action of
carbonic oxide on platinous chloride. It decomposes with an
explosion in contact with water. Carbon tetrachloride dissolves a
portion of this substance, and on evaporation gives crystals of
2PtCl_{2},3CO, whilst the compound PtCl_{2},2CO remains
undissolved. When fused and sublimed it gives yellow needles of
PtCl_{2},CO, and in the presence of an excess of carbonic oxide
PtCl_{2},2CO is formed. These compounds are fusible (the first at
250°, the second at 142°, and the third at 195°). In this case (as
in the double cyanides) combination takes place, because both
carbonic oxide and platinous chloride are unsaturated compounds
capable of further combination. The carbon tetrachloride solution
absorbs NH_{3} and gives PtCl_{2},CO,2NH_{3}, and
PtCl_{2},2CO,2NH_{3}, and these substances are analogous
(Foerster, Zeisel, Jörgensen) to similar compounds containing
complex amines (for instance, pyridine, C_{5}H_{5}N), instead of
NH_{3}, and ethylene, &c., instead of CO, so that here we have a
whole series of complex platino-compounds. The compound PtCl_{2}CO
dissolves in hydrochloric acid without change, and the solution
disengages all the carbonic oxide when KCN is added to it, which
shows that those forces which bind 2 molecules of KCN to PtCl_{2}
can also bind the molecule CO, or 2 molecules of CO. When the
hydrochloric acid solution of PtCl_{2}CO is mixed with a solution
of sodium acetate or acetic acid, it gives a precipitate of PtOCO,
_i.e._ the Cl_{2} is replaced by oxygen (probably because the
acetate is decomposed by water). This oxide, PtOCO, splits up into
Pt + CO_{2} at 350°. PtSCO is obtained by the action of
sulphuretted hydrogen upon PtCl_{2}CO. All this leads to the
conclusion that the group PtCO is able to assimilate X_{2} =
Cl_{2}, S, O, &c. (Mylius, Foerster, 1891). Pullinger (1891), by
igniting spongy platinum at 250°, first in a stream of chlorine,
and then in a stream of carbonic oxide, obtained (besides volatile
products) a non-volatile yellow substance which remained unchanged
in air and disengaged chlorine and phosgene gas when ignited; its
composition was PtCl_{6}(CO)_{2}, which apparently proves it to be
a compound of PtCl_{2} and 2COCl_{2}, as PtCl_{2} is able to
combine with oxychlorides, and forms somewhat stable compounds.

(_D_) The faculty of platinous chloride for forming stable
compounds with divers substances shows itself in the formation of
the compound PtCl_{2},PCl_{3} by the action of phosphorus
pentachloride at 250° on platinum powder (Pd reacts in a similar
manner, according to Fink, 1892). The product contains both
phosphorus pentachloride and platinum, whilst the presence of
PtCl_{2} is shown in the fact that the action of water produces
_chlorplatino-phosphorous acid_, PtCl_{2}P(OH)_{3}.

(_E_) After the cyanides, the _double salts_ of platinum _formed
by sulphurous acid_ are most distinguished for their stability and
characteristic properties. This is all the more instructive, as
sulphurous acid is only feebly energetic, and, moreover, in these,
as in all its compounds, it exhibits a dual reaction. The salts of
sulphurous acid, R_{2}SO_{3}, either react as salts of a feeble
bibasic acid, where the group SO_{3} presents itself as bivalent,
and consequently equal to X_{2}, or else they react after the
manner of salts of a monobasic acid containing the same residue,
RSO_{3}, as occurs in the salts of sulphuric acid. In sulphurous
acid this residue is combined with hydrogen, H(SO_{3}H), whilst in
sulphuric acid it is united with the aqueous residue (hydroxyl),
OH(SO_{3}H). These two forms of action of the sulphites appear in
their reactions with the platinum salts--that is to say, salts of
both kinds are formed, and they both correspond with the type
PtH_{2}X_{4}. The one series of salts contain PtH_{2}(SO_{3})_{2},
and their reactions are due to the bivalent residue of sulphurous
acid, which replaces X_{2}. The others, which have the composition
PtR_{2}(SO_{3}H)_{4}, contain sulphoxyl. The latter salts will
evidently react like acids; they are formed simultaneously with
the salts of the first kind, and pass into them. These salts are
obtained either by directly dissolving platinous oxide in water
containing sulphurous acid, or by passing sulphurous anhydride
into a solution of platinous chloride in hydrochloric acid. If a
solution of platinous chloride or platinous oxide in sulphurous
acid be saturated with sodium carbonate, it forms a white,
sparingly soluble precipitate containing
PtNa_{2}(SO_{3}Na)_{4},7H_{2}O. If this precipitate be dissolved
in a small quantity of hydrochloric acid and left to evaporate at
the ordinary temperature, it deposits a salt of the other type,
PtNa_{2}(SO_{3})_{2},H_{2}O, in the form of a yellow powder, which
is sparingly soluble in water. The potassium salt analogous to the
first salt, PtK_{2}(SO_{3}K)_{4},2H_{2}O, is precipitated by
passing sulphurous anhydride into a solution of potassium sulphite
in which platinous oxide is suspended. A similar salt is known for
ammonium, and with hydrochloric acid it gives a salt of the second
kind, Pt(NH_{4})_{2}(SO_{3})_{2},H_{2}O. If ammonio-chloride of
platinum be added to an aqueous solution of sulphurous anhydride,
it is first deoxidised, and chlorine is evolved, forming a salt of
the type PtX_{2}; a double decomposition then takes place with the
ammonium sulphite, and a salt of the composition
Pt(NH_{4})_{2}Cl_{3}(SO_{3}H) is formed (in a desiccator). The
acid character of this substance is explained by the fact that it
contains the elements SO_{3}H--sulphoxyl, with the hydrogen not
yet displaced by a metal. On saturating a solution of this acid
with potassium carbonate it gives orange-coloured crystals of a
potassium salt of the composition Pt(NH_{4})_{2}Cl_{3}(SO_{3}K).
Here it is evident that an equivalent of chlorine in
Pt(NH_{4})_{2})Cl_{4} is replaced by the univalent residue of
sulphurous acid. Among these salts, that of the composition
Pt(NH_{4})_{2})Cl_{2}(SO_{3}H)_{2},H_{2}O is very readily formed,
and crystallises in well-formed colourless crystals; it is
obtained by dissolving ammonium platinosochloride,
Pt(NH_{4})_{2}Cl_{4}, in an aqueous solution of sulphurous acid.
The difficulty with which sulphurous anhydride and platinum are
separated from these salts indicates the same basic character in
these compounds as is seen in the double cyanides of platinum. In
their passage into a complex salt, the metal platinum and the
group SO_{2} modify their relations (compared with those of
PtX_{2} or SO_{2}X_{2}), just as the chlorine in the salts KClO,
KClO_{3}, and KClO_{4} is modified in its relations as compared
with hydrochloric acid or potassium chloride.

(_F_) No less characteristic are the _platinonitrites_ formed by
platinous oxide. They correspond with nitrous acid, whose salts,
RNO_{2}, contain the univalent radicle, NO_{2}, which is capable
of replacing chlorine, and therefore the salts of this kind should
form a common type PtR_{2}(NO_{2})_{4}, and such a salt of
potassium has actually been obtained by mixing a solution of
potassium platinosochloride with a solution of potassium nitrite,
when the liquid becomes colourless, especially if it be heated,
which indicates the change in the chemical distribution of the
elements. As the liquid decolorises it gradually deposits
sparingly soluble, colourless prisms of the potassium salt
K_{2}Pt(NO_{2})_{4}, which does not contain any water. With silver
nitrate a solution of this salt gives a precipitate of silver
platinonitrite, PtAg_{2}(NO_{2})_{4}. The silver of this salt may
be replaced by other metals by means of double decomposition with
metallic chlorides. The sparingly soluble barium salt, when
treated with an equivalent quantity of sulphuric acid, gives a
soluble acid, which separates, under the receiver of an air-pump,
in red crystals; this acid has the composition
PtH_{2}(NO_{2})_{4}. To the potassium salt, K_{2}Pt(NO_{2})_{4},
there correspond (Vèzes, 1892) K_{2}Pt(NO_{2})_{4}Br_{2} and
K_{2}Pt(NO_{2})_{4}Cl_{2} and other compounds of the same type
K_{2}PtX_{6}, where X is partly replaced by Cl or Br and partly by
(NO_{2}), showing a transition towards the type of the double
salts like the platino-ammoniacal salts. (The corresponding double
sodium nitrite salt of cobalt is soluble in water, while the
K,NH_{4} and many other salts are insoluble in water, as I was
informed by Prof. K. Winkler in 1894).

In all the preceding complex compounds of Pt we see a common type
PtX_{2},2MX (_i.e._ of double salts corresponding to PtO) or
PtM_{2}X_{4} = Pt(MX_{2})_{2}, corresponding to Pt(HO)_{2} with
the replacement of O by its equivalent X_{2}. Two other facts must
also be noted. In the first place these X's generally correspond
to elements (like chlorine) or groups (like CN, NO_{2}, SO_{3},
&c.), which are capable of further combination. In the second
place all the compounds of the type PtM_{2}X_{4} are capable of
combining with chlorine or similar elements, and thus passing into
compounds of the types PtX_{3} or PtX_{4}.

Ammonia, like potassium cyanide, has the faculty of easily reacting with platinum dichloride, forming compounds similar to the platinocyanide and cobaltia compounds, which are comparatively stable. But as ammonia does not contain any hydrogen easily replaceable by metals, and as ammonia itself is able to combine with acids, the PtX_{2} plays, as it were, the part of an acid with reference to the ammonia. Owing to the influence of the ammonia, the X_{2} in the resultant compound will represent the same character as it has in ammoniacal salts; consequently, the ammoniacal compounds produced from PtX_{2} will be salts in which X will be replaceable by various other haloids, just as the metal is replaced in the cyanogen salts; such is the nature of the _platino-ammonium compounds_. PtX_{2} forms compounds with 2NH_{3} and with 4NH_{3}, and so also PtX_{4} gives (not directly from PtX_{4} and ammonia, but from the compounds of PtX_{2} by the action of chlorine, &c.) similar compounds with 2NH_{3} and with 4NH_{3}.[12]

[12] The platinum salt and ammonia, when once combined together, are no
longer subject to their ordinary reactions but form compounds
which are comparatively very stable. The question at once suggests
itself to all who are acquainted with these phenomena, as to what
is the relation of the elements contained in these compounds. The
first explanation is that these compounds are salts of ammonium in
which the hydrogen is partially replaced by platinum. This is the
view, with certain shades of difference, held by many respecting
the platino-ammonium compounds. They were regarded in this light
by Gerhardt, Schiff, Kolbe, Weltzien, and many others. If we
suppose the hydrogen in 2NH_{4}X to be replaced by bivalent
platinum (as in the salts PtX_{2}), we shall obtain
NH_{3} X
Pt
NH_{3} X
--that is, the compound PtX_{2},2NH_{3}. The compound with 4NH_{3}
will then be represented by a further substitution of the hydrogen
in ammonia by ammonium itself--_i.e._ as NH_{2}(NH_{4}X)_{2}Pt or
PtX_{2},4NH_{3}. A modification of this view is found in that
representation of compounds of this kind which is based on
atomicity. As platinum in PtX_{2} is bivalent, has two affinities,
and ammonia, NH_{3}, is also bivalent, because nitrogen is
quinquivalent and is here only combined with H_{3}, it is evident
what bonds should be represented in PtX_{2},2NH_{3} and in
PtX_{2},4NH_{3}. In the former, Pt(NH_{3}Cl)_{2}, the nitrogen of
each atom of ammonia is united by three affinities with H_{3}, by
one with platinum, and by the fifth with chlorine. The other
compound is Pt(NH_{3}.NH_{3}Cl)_{2}--that is, the N is united by
one affinity with the other N, whilst the remaining bonds are the
same as in the first salt. It is evident that this union or chain
of ammonias has no obvious limit, and the most essential fault of
such a mode of representation is that it does not indicate at all
what number of ammonias are capable of being retained by platinum.
Moreover, it is hardly possible to admit the bond between nitrogen
and platinum in such stable compounds, for these kinds of
affinities are, at all events, feeble, and cannot lead to
stability, but would rather indicate explosive and
easily-decomposed compounds. Moreover, it is not clear why this
platinum, which is capable of giving PtX_{4}, does not act with
its remaining affinities when the addition of ammonia to PtX_{2}
takes place. These, and certain other considerations which
indicate the imperfection of this representation of the structure
of the platino-ammonium salts, cause many chemists to incline more
to the representations of Berzelius, Claus, Gibbs, and others, who
suppose that NH_{3} is able to combine with substances, to adjoin
itself or pair itself with them (this kind of combination is
called 'Paarung') without altering the fundamental capacity of a
substance for further combinations. Thus, in PtX_{2},2NH_{3}, the
ammonia is the associate of PtX_{2}, as is expressed by the
formula N_{2}H_{6}PtX_{2}. Without enlarging on the exposition of
the details of this doctrine, we will only mention that it, like
the first, does not render it possible to foresee a limit to the
compounds with ammonia; it isolates compounds of this kind into a
special and artificial class; does not show the connection between
compounds of this and of other kinds, and therefore it essentially
only expresses the fact of the combination with ammonia and the
modification in its ordinary reactions. For these reasons we do
not hold to either of these proposed representations of the
ammonio-platinum compounds, but regard them from the point of view
cited above with reference to double salts and water of
crystallisation--that is, we embrace all these compounds under the
representation of compounds of complex types. The type of the
compound PtX_{2},2NH_{3} is far more probably the same as that of
PtX_{2},2Z--_i.e._ as PtX_{4}, or, still more accurately and
truly, it is a compound of the same type as PtX_{2},2KX or
PtX_{2},2H_{2}O, &c. Although the platinum first entered into
PtK_{2}X_{4} as the type PtX_{2}, yet its character has changed in
the same manner as the character of sulphur changes when from
SO_{2} the compound SO_{2}(OH)_{2} is obtained, or when KClO_{4},
the higher form, is obtained from KCl. For us as yet there is no
question as to _what_ affinities hold X_{2} and what hold 2NH_{3},
because this is a question which arises from the supposition of
the existence of different affinities in the atoms, which there is
no reason for taking as a common phenomenon. It seems to me that
it is most important _as a commencement_ to render clear the
analogy in the formation of various complex compounds, and it is
this analogy of the ammonia compounds with those of water of
crystallisation and double salts that forms the main object of the
primary generalisation. We recognise in platinum, at all events,
not only the four affinities expressed in the compound PtCl_{4},
but a much larger number of them, if only the _summation of
affinities_ is actually possible. Thus, in sulphur we recognise
not two but a much greater number of affinities; it is clear that
at least six affinities can act. So also among the analogues of
platinum: osmic anhydride, OsO_{4}, Ni(CO)_{4}, PtH_{2}Cl_{6}, &c.
indicate the existence of at least eight affinities; whilst, in
chlorine, judging from the compound KClO_{4} = ClO_{3}(OK) =
ClX_{7}, we must recognise at least seven affinities, instead of
the one which is accepted. The latter mode of calculating
affinities is a tribute to that period of the development of
science when only the simplest hydrogen compounds were considered,
and when all complex compounds were entirely neglected (they were
placed under the class of molecular compounds). This is
insufficient for the present state of knowledge, because we find
that, in complex compounds as in the most simple, the same
constant types or modes of equilibrium are repeated, and the
character of certain elements is greatly modified in the passage
from the most simple into very complex compounds.

Judging from the most complex platino-ammonium compounds
PtCl_{4},4NH_{3}, we should admit the possibility of the formation
of compounds of the type PtX_{4}Y_{4}, where Y_{4} = 4X_{2} =
4NH_{3}, and this shows that those forces which form such a
characteristic series of double platinocyanides
PtK_{2}(CN)_{4},3H_{2}O, probably also determine the formation of
the higher ammonia derivatives, as is seen on comparing--

PtCl_{2} NH_{3} Cl_{2} 3NH_{3}
Pt(CN)_{2} KCN KCN 3H_{2}O.

Moreover, it is obviously much more natural to ascribe the faculty
for combination with _n_Y to the whole of the acting
elements--that is, to PtX_{2} or PtX_{4}, and not to platinum
alone. Naturally such compounds are not produced with any Y. With
certain X's there only combine certain Y's. The best known and
most frequently-formed compounds of this kind are those with
water--that is, compounds with water of crystallisation. Compounds
with salts are double salts; also we know that similar compounds
are also frequently formed by means of ammonia. Salts of zinc,
ZnX_{2}, copper, CuX_{2}, silver, AgX, and many others give
similar compounds, but these and many other _ammonio-metallic_
saline compounds are unstable, and readily part with their
combined ammonia, and it is only in the elements of the platinum
group and in the group of the analogues of iron, that we observe
the faculty to form stable ammonio-metallic compounds. It must be
remembered that the metals of the platinum and iron groups are
able to form several high grades of oxidation which have an acid
character, and consequently in the lower degrees of combination
there yet remain affinities capable of retaining other elements,
and they probably retain ammonia, and hold it the more stably,
because all the properties of the platinum compounds are rather
acid than basic--that is, PtX_{n} recalls rather HX or SnX_{n} or
CX_{n} than KX, CaX_{2}, BaX_{2}, &c., and ammonia naturally will
rather combine with an acid than with a basic substance. Further,
a dependence, or certain connection of the forms of oxidation with
the ammonia compounds, is seen on comparing the following
compounds:

PdCl_{2},2NH_{3},H_{2}O PdCl_{2},4NH_{3},H_{2}O
PtCl_{2},2NH_{3} PtCl_{4},4NH_{3}
RhCl_{3},5NH_{3} RuCl_{2},4NH_{3},3H_{2}O
IrCl_{3},5NH_{3} OsCl_{2},4NH_{3},2H_{2}O

We know that platinum and palladium give compounds of lower types
than iridium and rhodium, whilst ruthenium and osmium give the
highest forms of oxidation; this shows itself in this case also.
We have purposely cited the same compounds with 4NH_{3} for osmium
and ruthenium as we have for platinum and palladium, and it is
then seen that Ru and Os are capable of retaining 2H_{2}O and
3H_{2}O, besides Cl_{2} and NH_{3}, which the compounds of
platinum and palladium are unable to do. The same ideas which were
developed in Note 35, Chapter XXII. respecting the cobaltia
compounds are perfectly applicable to the present case, _i.e._ to
the _platinia_ compounds or ammonia compounds of the platinum
metals, among which Rh and Ir give compounds which are perfectly
analogous to the cobaltia compounds.

Iridium and rhodium, which easily give compounds of the type
RX_{3}, give compounds (Claus) of the type IrX_{3},5NH_{3}, of a
rose colour, and RhX_{3},5NH_{3}, of a yellow colour. Jörgensen,
in his researches on these compounds, showed their entire analogy
with the cobalt compounds, as was to be expected from the periodic
system.

If ammonia acts on a boiling solution of platinous chloride in hydrochloric acid, it produces the green _salt of Magnus_ (1829), PtCl_{2},2NH_{3}, insoluble in water and hydrochloric acid. But, judging by its reactions, this salt has twice this formula. Thus, Gros (1837), on boiling Magnus's salt with nitric acid, observed that half the chlorine was replaced by the residue of nitric acid and half the platinum was disengaged: 2PtCl_{2}(NH_{3})_{2} + 2HNO_{3} = PtCl_{2}(NO_{3})_{2}(NH_{3})_{4} + 2PtCl_{2}. The Gros's salt thus obtained, PtCl_{2}(NO_{3})_{2}4NH_{3} (if Magnus's salt belongs to the type PtX_{2}, then Gros's salt belongs to the type PtX_{4}), is soluble in water, and the elements of nitric acid, but not the chlorine, contained in it are capable of easily submitting themselves to double saline decomposition. Thus silver nitrate does not enter into double decomposition with the chlorine of Gros's salt. Most instructive was the circumstance that Gros, by acting on his salt with hydrochloric acid, succeeded in substituting the residue of nitric acid in it by chlorine, and the chlorine thus introduced, easily reacted with silver nitrate. Thus it appeared that Gros's salt contained two varieties of chlorine--one which reacts readily, and the other which reacts with difficulty. The composition of Gros's first salt is PtCl_{2}(NH_{3})_{4}(NO_{3})_{2}; it may be converted into PtCl_{2}(NH_{3})_{4}(SO_{4}), and in general into PtCl_{2}(NH_{3})_{4}X_{2}.[13]

[13] Subsequently, a whole series of such compounds was obtained with
various elements in the place of the (non-reacting) chlorine, and
nevertheless they, like the chlorine, reacted with difficulty,
whilst the second portion of the X's introduced into such salts
easily underwent reaction. This formed the most important reason
for the interest which the study of the composition and structure
of the platino-ammonium salts subsequently presented to many
chemists, such as Reiset, Blomstrand, Peyrone, Raeffski, Gerhardt,
Buckton, Clève, Thomsen, Jörgensen, Kournakoff, Verner, and
others. The salts PtX_{4},2NH_{3}, discovered by Gerhardt, also
exhibited several different properties in the two pairs of X's. In
the remaining platino-ammonium salts all the X's appear to react
alike.

The quality of the X's, retainable in the platino-ammonium salts,
may be considerably modified, and they may frequently be wholly or
partially replaced by hydroxyl. For example, the action of ammonia
on the nitrate of Gerhardt's base, Pt(NO_{3})_{4},2NH_{3}, in a
boiling solution, gradually produces a yellow crystalline
precipitate which is nothing else than a _basic hydrate_ or
_alkali_, Pt(OH)_{4},2NH_{3}. It is sparingly soluble in water,
but gives directly soluble salts PtX_{4},2NH_{3} with acids. The
stability of this hydroxide is such that potash does not expel
ammonia from it, even on boiling, and it does not change below
130°. Similar properties are shown by the hydroxide
Pt(OH)_{2},2NH_{3} and the oxide PtO,2NH_{3} of Reiset's second
base. But the hydroxides of the compounds containing 4NH_{3} are
particularly remarkable. The presence of ammonia renders them
soluble and energetic. The brevity of this work does not permit
us, however, to mention many interesting particulars in connection
with this subject.

The salt of Magnus when boiled with a solution of ammonia gives the salt (of Reiset's first base) PtCl_{2}(NH_{3})_{4}, and this, when treated with bromine, forms the salt PtCl_{2}Br_{2}(NH_{3})_{4}, which has the same composition and reactions as Gros's salt. To Reiset's salts there corresponds a soluble, colourless, crystalline _hydroxide_, Pt(OH)_{2}(NH_{3})_{4}, having the properties of a powerful and very energetic _alkali_; it attracts carbonic anhydride from the atmosphere, precipitates metallic salts like potash, saturates active acids, even sulphuric, forming colourless (with nitric, carbonic, and hydrochloric acids), or yellow (with sulphuric acid), salts of the type PtX_{2}(NH_{3})_{4}.[14] The comparative stability (for instance, as compared with AgCl and NH_{3}) of such compounds, and the existence of many other compounds analogous to them, endows them with a particular chemical interest. Thus Kournakoff (1889) obtained a series of corresponding compounds containing thiocarbamide, CSN_{2}H_{4}, in the place of ammonia, PtCl_{2},4CSN_{2}H_{4}, and others corresponding with Reiset's salts. Hydroxylamine, and other substances corresponding with ammonia, also give similar compounds. The common properties and composition of such compounds show their entire analogy to the cobaltia compounds (especially for ruthenium and iridium) and correspond to the fact that both the platinum metals and cobalt occur in the same, eighth, group.

[14] Hydroxides are known corresponding with Gros's salts, which
contain one hydroxyl group in the place of that chlorine or haloid
which in Gros's salts reacts with difficulty, and these hydroxides
do not at once show the properties of alkalis, just as the
chlorine which stands in the same place does not react distinctly;
but still, after the prolonged action of acids, this hydroxyl
group is also replaced by acids. Thus, for example, the action of
nitric acid on Pt(NO_{3})_{2}Cl_{2},4NH_{3} causes the non-active
chlorine to react, but in the product all the chlorine is not
replaced by NO_{3}, but only half, and the other half is replaced
by the hydroxyl group: Pt(NO_{3})_{2}Cl_{2},4NH_{3} + HNO_{3} +
H_{2}O = Pt(NO_{3})_{3}(OH),4NH_{3} + 2HCl; and this is
particularly characteristic, because here the hydroxyl group has
not reacted with the acid--an evident sign of the non-alkaline
character of this residue. I think it may be well to call
attention to the fact that the composition of the
ammonio-metallosalts very often exhibits a correspondence between
the amount of X's and the amount of NH_{3}, of such a nature that
we find they contain either XNH_{3} or the grouping X_{2}NH_{3};
for example, Pt(XNH_{3})_{2} and Pt(X_{2}NH_{3})_{2},
Co(X_{2}NH_{3})_{3}, Pt(XNH_{3})_{4}, &c. Judging from this, the
view of the constitution of the double cyanides of platinum given
in Note 11 finds some confirmation here, but, in my opinion, all
questions respecting the composition (and structure) of the
ammoniacal, double, complex, and crystallisation compounds stand
connected with the solution of questions respecting the formation
of compounds of various degrees of stability, among which a theory
of solutions must be included, and therefore I think that the time
has not yet come for a complete generalisation of the data which
exist for these compounds; and here I again refer the reader to
Prof. Kournakoff's work cited in Chapter XXII., Note 35. However,
we may add a few individual remarks concerning the platinia
compounds.

To the common properties of the platino-ammonium salts, we must
add not only their _stability_ (feeble acids and alkalis do not
decompose them, the ammonia is not evolved by heating, &c.), but
also the fact that the ordinary reactions of platinum are
concealed in them to as great an extent as those of iron in the
ferricyanides. Thus neither alkalis nor hydrogen sulphide will
separate the platinum from them. For example, sulphuretted
hydrogen in acting on Gros's salts gives sulphur, removes half the
chlorine by means of its hydrogen, and forms salts of Reiset's
first base. This may be understood or explained by considering the
platinum in the molecule as covered, walled up by the ammonia, or
situated in the centre of the molecule, and therefore inaccessible
to reagents. On this assumption, however, we should expect to find
clearly-expressed ammoniacal properties, and this is not the case.
Thus ammonia is easily decomposed by chlorine, whilst in acting on
the platino-ammonium salts containing PtX_{2} and 2NH_{3} or
4NH_{3}, chlorine combines and does not destroy the ammonia; it
converts Reiset's salts into those of Gros and Gerhardt. Thus from
PtX_{2},2NH_{3} there is formed PtX_{2}Cl_{2},2NH_{3}, and from
PtX_{2},4NH_{3} the salt of Gros's base PtX_{2}Cl_{2},4NH_{3}.
This shows that the amount of chlorine which combines is not
dependent on the amount of ammonia present, but is due to the
basic properties of platinum. Owing to this some chemists suppose
the ammonia to be inactive or passive in certain compounds. It
appears to me that these relations, these modifications, in the
usual properties of ammonia and platinum are explained directly by
their mutual combination. Sulphur, in sulphurous anhydride,
SO_{2}, and hydrogen sulphide, SH_{2}, is naturally one and the
same, but if we only knew of it in the form of hydrogen sulphide,
then, having obtained it in the form of sulphurous anhydride, we
should consider its properties as hidden. The oxygen in magnesia,
MgO, and in nitric peroxide, NO_{2}, is so different that there is
no resemblance. Arsenic no longer reacts in its compounds with
hydrogen as it reacts in its compounds with chlorine, and in their
compounds with nitrogen all metals modify both their reactions and
their physical properties. We are accustomed to judge the metals
by their saline compounds with haloid groups, and ammonia by its
compounds with acid substances, and here, in the
platino-compounds, if we assume the platinum to be bound to the
entire mass of the ammonia--to its hydrogen and nitrogen--we shall
understand that both the platinum and ammonia modify their
characters. Far more complicated is the question why a portion of
the chlorine (and other haloid simple and complex groups) in
Gros's salts acts in a different manner from the other portion,
and why only half of it acts in the usual way. But this also is
not an exclusive case. The chlorine in potassium chlorate or in
carbon tetrachloride does not react with the same ease with metals
as the chlorine in the salts corresponding with hydrochloric acid.
In this case it is united to oxygen and carbon, whilst in the
platino-ammonium compounds it is united partly to platinum and
partly to the platino-ammonium group. Many chemists, moreover,
suppose that a part of the chlorine is united directly to the
platinum and the other part to the nitrogen of the ammonia, and
thus explain the difference of the reactions; but chlorine united
to platinum reacts as well with a silver salt as the chlorine of
ammonium chloride, NH_{4}Cl, or nitrosyl chloride, NOCl, although
there is no doubt that in this case there is a union between the
chlorine and nitrogen. Hence it is necessary to explain the
absence of a facile reactive capacity in a portion of the chlorine
by the conjoint influence of the platinum and ammonia on it,
whilst the other portion may be admitted as being under the
influence of the platinum only, and therefore as reacting as in
other salts. By admitting a certain kind of stable union in the
platino-ammonium grouping, it is possible to imagine that the
chlorine does not react with its customary facility, because
access to a portion of the atoms of chlorine in this complex
grouping is difficult, and the chlorine union is not the same as
we usually meet in the saline compounds of chlorine. These are the
grounds on which we, in refuting the now accepted explanations of
the reactions and formation of the platino-compounds, pronounce
the following opinion as to their structure.