Question

The hexaaquamanganese (II) ion contains five unpaired electrons while the hexacyano ion contains only one unpaired electron. Explain using Crystal Field Theory.

Answer 1

As we know that the basic ideas of crystal field theory involves the interaction between the transition metals elements and the ligands. Also the CFT describes the attraction effect between positive charge of metal which is a cation and negative charge of non-bonding electrons of ligand.

Complete solution:
Crystal field thoery’s fundamental premise is that interactions between metal and ligand are
entirely electrostatic in nature. The main ideas which basically made up the crystal field theory are:

1. Due to the static electric field and degeneracy of d-subshell or f-subshell is broken as the ligand reaches the central metal ion.
2. The d-electron which is closest to the ligand possesses a higher energy than the one which is farther away and this is due to repulsion between the electrons which results in splitting of d-orbitals.
3. The stability energy that results from the binding of ligands is known as the stabilisation energy of the crystal field (CFSE).
   Now, by considering the following rules, $Mn\left( {II} \right)$ possess the last subshell electronic configuration as $3{d^5}4{s^0}$ in case of hexaaquamanganese (II) ${[Mn{({H_2}O)_6}]^{2 - }}$ and water is approaching as ligand, which is basically a weak ligand and as we are aware that the weak ligands are not able to pair up the electron of $3d$ subshell , therefore, the electronic configuration will be distributed as ${t_2}{g^3}$ and $e{g^2}$ resulting in five unpaired electrons in the $3d$ subshell.
   Talking about the case of hexacyano ion ${[Mn{(CN)_6}]^{4 - }}$, the $C{N^ - }$ group is acting as a ligand and we know that it is a strong field ligand and hence it will help in pairing up the electrons in the d-orbitals and now the electronic configuration becomes ${t_2}{g^5}e{g^0}$ as the two ${t_2}g$ orbitals contains paired electrons and the remaining one contains an unpaired electron.

Note: Always remember that a strong ligand will always help in pairing up of electrons whereas a weak ligand will be unable to help in pairing of electrons in d-orbitals. Also remember that the cyano group is a strong field ligand and water is a weak field ligand along with carbonyl group.