(i) In $[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_{6]}^{+2}.$ the oxidation state of $\mathrm{Fe}$ is +2 . The outermost electronic configuration of $\mathrm{Fe}$ is $3{\mathit{d}}^{6}4{\mathit{s}}^{2}4{\mathit{p}}^0$ . HO is weak field ligand and does not cause pairing up of electrons.
In this complex, there are 4 unpaired electrons. Therefore, it involves $\mathit{s}{\mathit{p}}^3{\mathit{d}}^2$ hybridisation and is paramagnetic.
(ii) In $\left[\mathrm{Fe}{\left(\mathrm{CO}\right)}_5\right]$ , the oxidation state of $\mathrm{Fe}$ is zero. The outermost electronic configuration of $\mathrm{Fe}$ is $3{\mathit{d}}^{6}4{\mathit{s}}^{2}4{\mathit{p}}^0$. CO is strong field ligand, causes pairing up of $4\mathit{s}$ -electrons into the $3\mathit{d}$ -orbitals.In this complex, there is no unpaired electron.
Therefore, it involves dsp ${\mathit{p}}^3$ hybridisation and is diamagnetic.