I have read that H2O acts as a strong ligand in Co , Ni and Cu having +3 oxidation state. Is it true? Is it true then how sp3d2 hybridisation occurs in [Co(H2O)6]+3. Please Explain

H2O is undoubtedly a weak field ligand. However while dealing with complexes having central atom in higher oxidation state or of 2nd and 3rd transition series the nuclear pull of the central atom changes the scenario.

In simple words it can be said that H2O behaves as a strong field ligand with Co+3 and ions of 2nd and 3rd transition series.

Moreover, technically it’s incorrect to say that H2O will behave as strong ligand with any of the metals according to Crystal field theory, as there are many other factors that affect the hybridisation of a complex apart from the nature of the ligand. But to simplify the concept it’s widely used for jee level.