166 METAL-AMMONIA COMPOUNDS
Besides ammonia a number of other substances of similar nature, such as
ethylene diamine and pyridine, often take part in the formation of complex
cations.
A comparison of the composition of a large number of complex com-
pounds which have been carefully studied from different points of view has
shown that here, as in the case of the complex anions, the number of atoms
or atomic groups which are attached to a single metal atom is in a majority
of cases either four or six. Werner^1 accounts for this by assuming here also
that the constituents forming the complex are situated around the central
metal atom either in the four corners of a square or in the six corners of an
octahedron. The ever-recurring coordination numbers 4 and 6 would thus
be a measure of the available space around the central atom, but they are quite
independent of the valence of this atom. This space, which Werner desig-
nates as the "inner sphere" is characterized by the fact that no components
can be dislodged from it by electrolytic dissociation and that the whole is
able to take part in chemical reactions exactly in the same way as an indi-
vidual metal atom. The valence of the complex is equal to the difference
between that of the central atom and the valence of the attached acid radi-
cals. If the valences of the acid residues are sufficient to satisfy that of the
central atom, then the total valence of the complex is zero, i.e. the substance
is a non-electrolyte, e.g. trinitrito-triammine-cobalt. (No. 139.)
Nomenclature. Before the constitution of the metal-ammonia com-
pounds was known certain series were designated according to their external
characteristics (e.g. color, in roseo salts and purpureo salts) while others
were named after the discoverer (e.g. Magnus' salt). Werner, after formu-
lating his theory, proposed a rational system of nomenclature which is now
quite generally adopted. According to this system the names of the con-
stituents belonging to the complex cation are placed before the name of the
metal and are arranged in the following order: first the acid radicals; then
any groups which behave in the same manner as ammonia; and lastly, just
preceding the metal, the ammonia itself. The acid radicals are given the
suffix "o" while ammonia is designated by the term "ammine," and water
contained in the complex is designated as aquo. The number of each kind of
constituent is indicated by the appropriate prefix, di, tri, etc. The com-
plete name of the cation is commonly printed as a single word. The anions are
designated in the usual way and follow the name of the cation. Further
particulars will be evident from a study of the examples in the accompanying
tables, pp. 167 and 168.
Isomerism. It is apparent from the tables on the following two pages
that in certain cases two compounds exist which have the same empirical
composition, and the same molecular weight, but still have different prop-
erties (isomerism). The existence of such isomers leads here, as in all similar
cases, to the assumption that there is a definite spatial arrangement of the
various parts of the molecule and gives justification for the arbitrary assump-
(^1) Neuere Anschauungen auf dem Gebiete der anorganischen Chemie, 4th
Ed. Braunschweig 1919.