${x}+6 \times(-1)=-4$
${x}=+2$
${Co}^{2+}:[{Ar}] 3 {~d}^{7}$
and ${CN}^{-}$is a strong field ligand which can pair electron of central atom.
It has one unpaired electron ( $n$ ) in $4d-$subshell.So spin only magnetic moment $(\mu)=\sqrt{n(n+2)}\, B . M$
where ${n}=$ number of unpaired electrons.
$\mu=\sqrt{3} \,{~B} . {M} \quad \mu=1.73\, {BM}$
$(i) [pt\ (en)\ Cl_2]$
$(ii)\ [pt (en)_2]\ Cl_2$
$ (iii)\ [pt (en)_2 Cl_2]\ Cl_2$
$(iv)\ [pt (CH_3)_2 Cl_2]$
(પ.ક્ર. : $Fe=26, Co= 27, Ni =28, Pt= 78$)