SILVER-AMMONIA SULPHATE. 169
tion that the constutuents of the "inner sphere" which surrounds the central
metal atom are actually situated at the corners of a square or of an octahedron.
Two different pairs of substituents are manifestly capable of two different
arrangements at the corners of a square;
NH3X /Cl NH,v, /ClNH 3 X XC1 CK XNH 3
1.2 or cis-position 1.3 or trans-position.
If, however, three substituents in a square are the same and the fourth
different, such an isomerism is impossible. The actual lack of isomers in the
latter case and their existence in the former, strongly support the above
assumption.
Similarly, the isomerism of the two dinitritotetramminecobaltic series is
explained by the use of the corresponding octahedral formulas (cf. No. 138).
As has already been stated, the external valence of the complex is equal to
the difference between the valence of the metal and the total valence of the
acid radicals contained in the complex. This external valence may become
zero, or even negative when the valence of the acid part of the complex
preponderates. In the following series of seven cobalt compounds, the valence
of the complex changes progressively from three positive to three negative:
[Co(NH 3 ) 6 ]Cl 3 [Co(NH 3 ) 6 NO 2 ]Cl 2 [Co(NH 3 ) 4 (NO 2 ) 2 ]Cl [Co(NH 3 ) 3 (NO 2 )J
K[Co(NH 3 ) 2 (NO 2 ) 4 ] (No. 115) K 2 [Co(NH 3 )(NO 2 ) 5 ]
K 3 [Co(NOa) 6 ] (No. 114).
The binding power for potassium, or in other words the affinity for nega-
tive electrons, is occasioned by the valences of the nitrite radicals. The bind-
ing of these nitrite groups within the complex takes place in part by means of
the secondary valences (cf. pp. 141 and 155).
2 ^ ,..-NO 2 -l-K
N0 2 -Co-N0 2 - -K
^ --NO 2 -J-K
so that of the six nitrite groups attached to the cobalt, three are held by the
principal valences and three by the secondary valences of the metal. Inas-
much as the secondary valences are weaker than the principal ones, this view
corresponds well with the formation of potassium cobaltinitrite from potas-
sium nitrite and cobaltic nitrite, and with its manner of ionizing. For
the further development of this theory, which leads to the conception of
"indirect combination," the work of Werner already cited should be consulted.- Silver-ammonia Sulphate [Ag(NH 3 ) 2 ] 2 SO 4.
Diamminesilver sulphate can be prepared in the solid form although this
is not possible with the corresponding chloride (cf. p. 164).
Treat 10 g. of silver nitrate with 31 c.c. of 2-normaI sulphuric
acid and heat the mixture in a small evaporating dish on an airbath, until no more acid vapors are given off. While still hot