Question:
Specify the oxidation numbers of the metals in the following coordination entities:
(i) [Co(H2O)(CN)(en)2]2+
(ii) [CoBr2(en)2]+
(iii) [PtCl4]2−
(iv) K3[Fe(CN)6]
(v) [Cr(NH3)3Cl3]
Solution:
(i) $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)(\mathrm{CN})(\mathrm{en})_{2}\right]^{2+}$
Let the oxidation number of Co be x.
The charge on the complex is +2.
(ii) 
Let the oxidation number of Pt be x.
The charge on the complex is −2.
x + 4(−1) = −2
x = + 2
(iv) 
