'Spin only' magnetic moment of Ni in $\left[\mathrm{Ni}{\left(\mathrm{dmg}\right)}_2\right]$ is the same as that found in 

 

$\mathrm{Ni} [\mathrm{Ni} {\left(\mathrm{dmg}\right)}_2]$ complex is square planar and diamagnetic. Pt (II) have $5{\mathrm{d}}^8$ configuration. The complex is square planar and therefore, diamagnetic. So, $\mathrm{\mu B}.\mathrm{M}=0.$

This is same as the $\mathrm{Mn}\text{ in }{\left[{\mathrm{MnO}}_4\right\}}^{-}$ and $\mathrm{Pt}\text{ in }\left[\mathrm{Pt}{\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\mathrm{Br}}_2\right]$.