In the p-block, it’s a well-known general trend that down the group, the homonuclear diatomic bond dissociation energies decrease (generally). E.g in group 14:
C-C —> 607 kJ/mol Si-Si —> 327 kJ/mol Ge-Ge —> 274 kJ/mol
The common explanation of this trend is that as the orbitals increase in size, 2p —> 3p —> 4p, they become more diffuse and worse orbital overlap is achieved, leading to a weaker bond.
However in T.M complexes it seems like the reverse is true. Complexes of the 4d and 5d metals are more likely to be low-spin, owing to the large splitting parameter achieved by stronger bonding to the ligands. This would suggest more effective overlap of 4d and 5d orbitals with the ligands than 3d, even if the ligands themselves are 2p atoms.
So my question is, why does orbital overlap seem to be more effective down the group in d-block elements, than in p-block elements, where orbital overlap clearly worsens?