Cobaltic salts, ammino-derivatives

The trinitro-triammine cobalt has practically no conductivity.3 Werner s theory is further supported by the fact that by the introduction of a fourth molecule of ammonia into the triacido-triammine compound the solution becomes once more conducting, as one (N02) group is displaced from the co-ordination complex. The eobalt-ammino-compounds, therefore, containing fewer than three ammino-radicles, contain non-ionisable acidic radicle, and those containing more than three contain ionisable acidic radicles. The generalisation made in connection with the triammino-eompounds led, therefore, to the establishment of the constitution of other ammino-derivatives, and also to the constitution of some of the ammino-salts of divalent and tetravalent metals. 

Nickel, although closely resembling cobalt, shows much less tendency to form complex radicles, and there is no long series of stable ammino-nickel salts as in the case of cobalt. Further, the stable series of ammino-derivatives of cobalt are those in which the metal is trivalent, whereas in the case of nickel the niekelie salts are unknown, and the complex ammino-derivatives are additive compounds of nickelous salts. 

All three metals of this group give rise to ammino-derivatives the compositions of which, however, differ considerably. The ammino-derivatives of ruthenium mostly contain a nitroso-group as well as ammonia the ammino-derivatives of rhodium salts closely resemble the cobalt -ammines and the ammino-derivatives of palladium salts correspond to the ammino-derivatives of platinum salts. 

Ammonia unites readily with iridium salts, giving rise to complex ammino-derivatives. The first compounds described appear to be ammines analogous to those of palladium and platinum, to which they were compared by Berzelius 8 and Skoblikoff.4 A further series were described by Claus 5 wliich he represented like those of ammino-rhodium salts, as they bore a marked resemblance to these. After Jorgensen had established the constitution of the ammines of rhodium, cobalt, and chromium salts, Palmaer gave similar constitution to the iridium compounds. 

In these the metal is divalent, tetravalent, and trivalent respectively. The ammino-iridous and the ammino-iridic salts correspond to the ammino-derivatives of palladium and platinum, whilst those of the sesqui-salts are analogous to the ammino-derivatives of cobalt, chromium, and rhodium. 

Platinum forms both platinous and platinie salts, in which the metal is divalent and tetravalent respectively. Both series of salts are capable of uniting with ammonia, forming complex ammines. The co-ordination number in the platinous series is four and in the platinie series six. The latter series correspond in many respects to the chromic and cobaltic ammino-salts, but as the metal is tetravalent, the maximum number of radicles outside the complex is four instead of three. Also, the ammino-bases from which the salts are derived are much more stable than those of chromium or cobalt. 

In compiling this volume, The Metal-Ammines, ail endeavour has been made to collect the main points relating to this large class of substances. The ammino-derivatives of chromic and cobaltic salts are well known and are included in most text-books on inorganic chemistry, whilst those of the other metals are barely touched upon. The amniincs are treated in the order in which the metals occur in the periodic system, thus keeping this volume in line with the others of the series. 

Ammino-derivatives op Cobalt Salts—Cobaltous Salt Ammines—Cobaltic Salt Ammines—Mononuclear Cobalt-ammines containing One Atom of Cobalt in the Molecule—Cobaltic Salts with Trivalent Cation—Cobalt-ammines Containing Divalent Cation—Cobalt-ammines containing Monovalent Cation—Cobalt-ammines consisting of Non-dissociable Complex— Cobalt-ammines containing Monovalent Anion—Cobalt Salts containing Trivalent Anion—Polynuclear Cobalt-ammines containing Two or more Cobalt Atoms in the Molecule—Cobalt-ammines of Unknown Constitution— Ionisation Metamerism—Polymerisation Isomerism—Valency Isomerism —Co-ordination Position Isomerism—Isomerism due to Asymmetric Cobalt Atoms. 

The more important simple derivatives of cobalt are divalent, the metal only yielding stable trivalent salts in conjunction with other metallic derivatives, as, for example, the cobalti-nitrites and eobalii-cyanides, or in the complex ammino derivatives. Rhodium and iridium function almost exclusively as trivalent metals in their salts. 

Cobalt, rhodium, and iridium are also characterised by their power of yielding complex ammino derivatives, and in this manner resemble palladium and platinum. These derivatives are a remarkable series of substances entirely distinct in most of their properties from the more usual inorganic salts of the metals in question, and are dealt with separately in Volume X of this Series. 

General Properties of Compounds of Cobalt.—Cobalt, in its salts, behaves both as a trivalent and a divalent element, although the trivalent salts are unstable except in the case of double salts. The colour of cobalt salts varies greatly according to the degree of hydration, but it is usually pink or blue. Cobalt salts absorb nitric oxide,1 and with ammonia readily yield complex ammino derivatives. 

---