NO2 and M = one
atomic proportion of a monovalent metal, or the equivalent quantity
of a divalent metal.

Triammine Series, [Co(NH3)3]X3. Here X = Cl, NO3, NO2, 1/2SO4, &c.

Tetrammine Series. This group may be divided into the

Praseo-salts [R2Co(NH3)4]X, where X = Cl.

Croceo-salts [(NO2)2Co(NH3)4]X, which may be considered as a
subdivision of the praseo-salts.

Tetrammine purpureo-salts [RCo(NH3)4.H2O]X2.

Tetrammine roseo-salts [Co(NH3)4.(H2O)2]X3.

Fuseo-salts [Co(NH3)4]OH.X2.

Pentammine Series.

Pentammine purpureo-salts [R.Co(NH3)5]X2 where X = Cl, Br, NO3,
N02, 1/2SO4, &c.

Pentammine roseo-salts [Co(NH3)5.H2O]X2.

Hexammine or Luteo Series [Co(NH3)6]X3.

The hexammine salts are formed by the oxidizing action of air on
dilute ammoniacal solutions of cobaltous salts, especially in presence
of a large excess of ammonium chloride. They form yellow or
bronze-coloured crystals, which decompose on boiling their aqueous
solution. On boiling their solution in caustic alkalis, ammonia is
liberated. The pentammine purpureo-salts are formed from the
luteo-salts by loss of ammonia, or from an air slowly oxidized
ammoniacal cobalt salt solution, the precipitated luteo-salt being
filtered off and the filtrate boiled with concentrated acids. They are
violet-red in colour, and on boiling or long standing with dilute
acids they pass into the corresponding roseo-salts.

The pentammine nitrito salts are known as the xanthocobalt salts and
have the general formula [NO2.Co.(NH3)5]X2. They are formed by the
action of nitrous fumes on ammoniacal solutions of cobaltous salts, or
purpureo-salts, or by the mutual reaction of chlorpurpureo-salts and
alkaline nitrites. They are soluble in water and give characteristic
precipitates with platinic and auric chlorides, and with potassium
ferrocyanide. The pentammine roseo-salts can be obtained from the
action of concentrated acids, in the cold, on air-oxidized solutions
of cobaltous salts. They are of a reddish colour and usually
crystallize well; on heating with concentrated acids are usually
transformed into the purpureo-salts. Their alkaline solutions liberate
ammonia on boiling. They give a characteristic pale red precipitate
with sodium pyrophosphate, soluble in an excess of the precipitant;
they also form precipitates on the addition of platinic