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.

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