Why is d3 orbital more stable in water than d5?

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Lets first take the case of the d3 compound. The no.of orbitals in

the 3d shell is 5. If three electrons occupy three orbitals then

there are two free orbitals.Therefore According to Valence bond

theory the six water ligands will use the two inner d orbitals the

outer s and the p orbitals to form an inner orbital complex with

hybrisation d2sp3.

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In the second case we have the d5 compund. Since there are five

electrons in the d subshell the five electrons singly occupy all

the five d orbitals. Here 's where the concept of the weak ligand

comes in. Since water is a weak ligand it cannot force pairing of

the unpaired d electrons to make room for an inner orbital complex.

Thus it has to use the outer d orbital to form an outer orbital

complex with hybridisation of sp3d2.

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Since the Inner orbital (low spin) complex is more stable than the

outer orbital (high spin) complex. Thus d3 configuration is more

stable than d5 configuration in aqueous medium.