(i) In $[\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6]^{+2}.$ the oxidation state of $\mathrm{Fe}$ is +2 . The outermost electronic configuration of $\mathrm{Fe}$ is $3 d^6 4 s^2 4 p^0$ . HO is weak field ligand and does not cause pairing up of electrons. In this complex, there are 4 unpaired electrons. Therefore, it involves $s p^3 d^2$ hybridisation and is paramagnetic. (ii) In $[\mathrm{Fe}(\mathrm{CO})_5]$ , the oxidation state of $\mathrm{Fe}$ is zero. The outermost electronic configuration of $\mathrm{Fe}$ is $3 d^6 4 s^2 4 p^0$. CO is strong field ligand, causes pairing up of $4 s$ -electrons into the $3 d$ -orbitals.In this complex, there is no unpaired electron. Therefore, it involves dsp $p^3$ hybridisation and is diamagnetic.