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Strong ligand-to-metal pi donation is favoured by 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.

Ultrashort metal–metal contacts are favoured when the valence configuration fills strongly metal–metal-bonding orbitals while leaving their antibonding partners empty. Early or middle transition metals in low oxidation states can retain enough d electrons for multiple bonding without the unavoidable antibonding occupation of a more filled d shell. Good radial and angular overlap, a compatible metal–metal axis, and low competing ligand-field stabilization all matter; a short contact should be interpreted together with an orbital/electron-counting model, not automatically called a high-order bond.

Ligands should support a geometry in which the relevant d orbitals can overlap and protect the low-valent pair from oligomerization or oxidative attack. Bulky aryl-substituted bridging amidinates can provide this protection while maintaining an appropriate small coordination number. Strongly coordinating axial ligands or inappropriate pi interactions can compete with metal–metal bonding, especially its weaker delta components. For quadruple or quintuple bonds, retaining two appropriately oriented delta-capable orbitals is particularly important; a geometry optimized only for metal–ligand bonding can remove that advantage.

Ligand bridges need not literally squeeze two otherwise unbound metals together. Comparing bond lengths across oxidation states and across related ligands helps distinguish electronic multiple bonding from enforced steric compression. The lantern-type dichromium experiment in (ii) is informative because reduction changes the electron occupation within essentially the same three-bridge supporting framework.^[ref]

The group-7 transition metal with atomic number 75.

A transition metal has an incompletely filled d subshell in its atom or a relevant cation. Its metal–ligand orbital interactions support varied oxidation states, magnetic states and coordination geometries.