Coordination Compounds 2 Question 2
####2. Complete removal of both the axial ligands (along the $z$-axis) from an octahedral complex leads to which of the following splitting patterns? (relative orbital energies not on scale).
(2019 Main, 12 April I)
(b)
(d) $E$
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Solution:
- Complete removal of both the axial ligands (along the $z$-axis) from an octahedral complex leads to the following splitting pattern.
The single electron in the $d _{x^{2}-y^{2}}$ orbital is being repelled by four ligands, while the electron in the $d _{z^{2}}$ orbital is only being repelled by two ligands. Thus, the energy of the $d _{x^{2}-y^{2}}$ increases relative to that of $d _{z _2}$. A more stable arrangement arises when both the $e _g$ electrons pair up and occupy the lower energy $d _{z^{2}}$ orbital. This leaves the $d _{x^{2}-y^{2}}$ orbital empty.
Thus, four ligands can now approach along $+x,-x,+y$ and $-y$ directions without any difficulty as $d _{x^{2}-y^{2}}$ orbital is empty. However, ligands approaching along $+z$ and $-z$ directions meet very strong repulsive forces from filled $d _{z^{2}}$ orbitals. Thus, only four ligands succeed in bonding to the metal. A square planar complex is formed, the attempt to form an octahedral complex being unsuccessful.
