why d₃ state is more stable than d₅ in aquous medium?? what is feature in this weak ligand water to cause this???

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.

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.

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.