This is because, three electrons enter $t_{2 g}$ level and are unpaired. When fourth
electron is present its position (in $t_{2 g}$ or $-g$ level) will be determined by the
strength of the ligand field.
$(i) [pt\ (en)\ Cl_2]$
$(ii)\ [pt (en)_2]\ Cl_2$
$ (iii)\ [pt (en)_2 Cl_2]\ Cl_2$
$(iv)\ [pt (CH_3)_2 Cl_2]$
[આપેલ છે :પરમાણ્વીય દળ:-$Cr =52$ $amu$ અને $Cl =35\, amu ]$