The number of ‘d’ electrons in  Fe²⁺ is not equal to that of
 

The electronic configuration of Fe atom is

$\mathrm{Fe}\left[\mathrm{Ar}\right]4{\mathrm{s}}^{2}3{\mathrm{d}}^6$

${\mathrm{Fe}}^{+2}\left[\mathrm{Ar}\right]4{\mathrm{s}}^{0}3{\mathrm{d}}^6$ [6 electrons in d -orbital]

The electronic configuration of Cl atom is

$\mathrm{Cl} = 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^5$[11 electrons in p -orbital]

The chlorine atom's p orbital can hold the (6+5 = 11) electrons. The six electrons in the d-orbital of the Fe²⁺ atom are distinct from the p-electrons in the chlorine atom.