Chapter XXIII: The Platinum Metals (3)

In characterising the platino-ammonium compounds, it is necessary
to bear in mind that compounds which already contain PtX_{4} do
not combine directly with NH_{3}, and that such compounds as
PtX_{4},4NH_{3} only proceed from PtX_{2}, and therefore it is
natural to conclude that those affinities and forces which cause
PtX_{2} to combine with X_{2} also cause it to combine with
2NH_{3}. And having the compound PtX_{2},2NH_{3}, and supposing
that in subsequently combining with Cl_{2} it reacts with those
affinities which produce the compounds of platinic chloride,
PtCl_{4}, with water, potassium chloride, potassium cyanide,
hydrochloric acid, and the like, we explain not only the fact of
combination, but also many of the reactions occurring in the
transition of one kind of platino-ammonium salts into another.
Thus by this means we explain the fact that (1) PtX_{2},2NH_{3}
combines with 2NH_{3}, forming salts of Reiset's first base; (2)
and the fact that this compound (represented as follows for
distinctness), PtX_{2},2NH_{3},2NH_{3}, when heated, or even when
boiled in solution, again passes into PtX_{2},2NH_{3} (which
resembles the easy disengagement of water of crystallisation,
&c.); (3) the fact that PtX_{2},2NH_{3} is capable of absorbing,
under the action of the same forces, a molecule of chlorine,
PtX_{2},2NH_{3},Cl_{2}, which it then retains with energy, because
it is attracted, not only by the platinum, but also by the
hydrogen of the ammonia; (4) the fact that this chlorine held in
this compound (of Gerhardt) will have a position unusual in salts,
which will explain a certain (although very feebly-marked)
difficulty of reaction; (5) the fact that this does not exhaust
the faculty of platinum for further combination (we need only
recall the compound PtCl_{4},2HCl,16H_{2}O), and that therefore
both PtX_{2},2NH_{3},Cl_{2} and PtX_{2},2NH_{3},2NH_{3} are still
capable of combination, whence the latter, with chlorine, gives
PtX_{2},2NH_{3},2NH_{3},Cl_{2}, after the type of PtX_{4}Y_{4}
(and perhaps higher); (6) the fact that Gros's compounds thus
formed are readily reconverted into the salts of Reiset's first
base when acted on by reducing agents; (7) the fact that in Gros's
salts, PtX_{2},2NH_{3}(NH_{3}X)_{2}, the newly-attached chlorine
or haloid will react with difficulty with salts of silver, &c.,
because it is attached both to the platinum and to the ammonia,
for both of which it has an attraction; (8) the fact that the
faculty for further combination is not even yet exhausted in the
type of Gros's salts, and that we actually have a compound of
Gros's chlorine salt with platinous chloride and with platinic
chloride; the salt PtSO_{4},2NH_{3},2NH_{3},SO_{4} combines
further also with H_{2}O; (9) the fact that such a faculty of
combination with new molecules is naturally more developed in the
lower forms of combination than in the higher. Hence the salts of
Reiset's first base--for example, PtCl_{2},2NH_{3},2NH_{3}--both
combine with water and give precipitates (soluble in water but not
in hydrochloric acid) of double salts with many salts of the heavy
metals--for example, with lead chloride, cupric chloride, and also
with platinic and platinous chlorides (Buckton's salts). The
latter compounds will have the composition
PtCl_{2},2NH_{3},2NH_{3},PtCl_{2}--that is, the same composition
as the salts of Reiset's second base, but it cannot be identical
with it. Such an interesting case does actually exist. The first
salt, PtCl_{2},4NH_{3},PtCl_{2}, is green, insoluble in water and
in hydrochloric acid, and is known as _Magnus's salt_, and the
second, PtCl_{2},2NH_{3}, is Reiset's yellow, sparingly soluble
(in water). They are polymeric, namely, the first contains twice
the number of elements held in the second, and at the same time
they easily pass into each other. If ammonia be added to a hot
hydrochloric acid solution of platinous chloride, it forms the
salt PtCl_{2},4NH_{3}, but in the presence of an excess of
platinous chloride it gives Magnus's salt. On boiling the latter
in ammonia it gives a colourless soluble salt of Reiset's first
base, PtCl_{2},4NH_{3}, and if this be boiled with water, ammonia
is disengaged, and a salt of Reiset's second base,
PtCl_{2},2NH_{3}, is obtained.

A class of platino-ammonium isomerides (obtained by Millon and
Thomsen) are also known. Buckton's salts--for example, the copper
salt--were obtained by them from the salts of Reiset's first base,
PtCl_{2},4NH_{3}, by treatment with a solution of cupric chloride,
&c., and therefore, according to our method of expression,
Buckton's copper salt will be PtCl_{2},4NH_{3},CuCl_{2}. This salt
is soluble in water, but not in hydrochloric acid. In it the
ammonia must be considered as united to the platinum. But if
cupric chloride be dissolved in ammonia, and a solution of
platinous chloride in ammonium chloride is added to it, a violet
precipitate is obtained of the same composition as Buckton's salt,
which, however, is insoluble in water, but soluble in hydrochloric
acid. In this a portion, if not all, of the ammonia must be
regarded as united to the copper, and it must therefore be
represented as CuCl_{2},4NH_{3},PtCl_{2}. This form is identical
in composition but different in properties (is isomeric) with the
preceding salt (Buckton's). The salt of Magnus is intermediate
between them, PtCl_{2},4NH_{3},PtCl_{2}; it is insoluble in water
and hydrochloric acid. These and certain other instances of
isomeric compounds in the series of the platino-ammonium salts
throw a light on the nature of the compounds in question, just as
the study of the isomerides of the carbon compounds has served and
still serves as the chief cause of the rapid progress of organic
chemistry. In conclusion, we may add that (according to the law of
substitution) we must necessarily expect all kinds of intermediate
compounds between the platino and analogous ammonia derivatives on
the one hand, and the complex compounds of nitrous acid on the
other. Perhaps the instance of the reaction of ammonia upon osmic
anhydride, OsO_{4}, observed by Fritsche, Frémy, and others, and
more fully studied by Joly (1891), belongs to this class. The
latter showed that when ammonia acts upon an alkaline solution of
OsO_{4} the reaction proceeds according to the equation: OsO_{4} +
KHO + NH_{3} = OsNKO_{3} + 2H_{2}O. It might be imagined that in
this case the ammonia is oxidised, probably forming the residue of
nitrous acid (NO), while the type OsO_{4} is deoxidised into
OsO_{2}, and a salt, OsO(NO)(KO), of the type OsX_{4} is formed.
This salt crystallises well in light yellow octahedra. It
corresponds to _osmiamic acid_, OsO(ON)(HO), whose anhydride
[OsO(NO)]_{2}, has the composition Os_{2}N_{2}O_{5}, which equals
2Os + N_{2}O_{5} to the same extent as the above-mentioned
compound PtCO_{2} equals Pt + CO_{2} (_see_ Note 11).