Question

In the complex potassium octacyanomolybdate (IV), the co-ordination number of Mo is:

• A. 4
• B. 6
• C. 8
• D. 2

Answer 1

The correct option is C 8

Complex is $K_{4}Mo(CN{)}_8{H}_{2}O$.
Coordination no. = No. of ligands attached to central metal

Mo exists in +4 oxidation state and since 4 K+ are present outside the coordination sphere this implies the charge on coordination sphere is -4 only possible when all 8 $C{N}^{-}$ ions are inside coordination sphere.