= Solution
Strong ligand-to-metal <pi donation> is favoured by \b[an occupied ligand <p orbital> and a suitable empty metal <d orbital>] of matching symmetry, with reasonably similar energies and good spatial overlap. Small donor atoms such as O or N can supply short bonds and efficient overlap; oxo, imido and nitrido ligands are familiar examples. Early <transition metals> in high <oxidation states>, often $d^0$, are particularly favourable because the metal acceptor orbitals are vacant and relatively low in energy. More spatially extended $4d$ and $5d$ orbitals can strengthen the overlap.
Metal-to-ligand <pi backbonding> is the complementary case: electron-rich, low-oxidation-state metals donate occupied <d orbitals> into low-lying ligand <antibonding orbitals>, as for CO. In both directions, filling a metal–ligand pi-antibonding orbital offsets the multiple-bond stabilization. The relevant prediction is orbital occupancy and overlap, rather than the presence of a formal double-bond line in a drawing.
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