Question

In splitting diagram of trigonal bipyramid geometry, why does z2 orbital have the highest energy and why does the energy decrease by this order?

z2 > xy,x2-y2 > xz,yz

Answer 1

According to crystal field theory trigonal pyramidal complex in coordinate complexes, when ligands approach towards metal ions, the valence d-orbitals of metal ions splits into t2g and eg levels according to their energy levels. The electrons of ligands with low energy goes to t2g and high energy level is dz^2. The splitting of orbitals in the presence of ligand forms:

a single energy level containing only

a twice-degenerate energy level containing and

a twice-degenerate energy level consisting of and  

This is because and orbitals are at perpendicular angle to the Z axis and differ only by their alignment. The orbital having lobes pointing along the coordinate axis while orbital lobes point in-between the coordinate axis. That is the reason they have similar energy at the time of splitting.

According to the crystal field model the has a central ‘hoop’ that is pointing towards the ligands thus has the highest energy due to repulsion force. In the xy plane while the and have a nodal plane thus they should have a lower energy.

You would better understand if you correlate with d-orbital geometry diagram.

Because of these coordination geometry of d-orbitals around the principal axis the order of energy is : d