In the compound k[co (c2o4) 2 (h2o) 2] (where c2o42 - = oxalate) the oxidation number and coordination number of cobalt are, respectively

Let's Assign Symbols to molecules like,

C₂O₄ = X

and

H₂O = Y

Then,

K [ Co (X) ₂ (Y) ₂ ]

As, Potassium (K) has a O. N = + 1

To neutralize, the coordination sphere must have - 1 oxidation number.

So,

[ Co (X) ₂ (Y) ₂ ] = - 1

As,

O. N of X = - 2

Then

O. N of (X) ₂ = - 4

Also,

O. N of H₂O is zero as it is neutral, So,

[Co - 4 + 0 ] = - 1

Or,

Co = - 1 + 4

Co = + 3

Result:

Oxidation Number of Coordination Sphere is - 1 and Oxidation Number of Cu is + 3.