= Solution
The principal electronic variable is the net population of <metal>–<metal> <bonding molecular orbitals> relative to <antibonding molecular orbitals>. For otherwise comparable complexes, a larger <bond order> generally gives a shorter <metal-metal bond>; removing an antibonding <electron> can shorten a bond even though the total <electron> count falls. Different orbital symmetries also matter: sigma, pi and delta overlaps respond differently to distance and rotation of the <metal> fragments.
<metal> size, <oxidation state> and radial orbital extent set the underlying distance scale. Comparing a first-row <metal> with a larger heavier congener does not isolate <bond order>. Increasing positive charge often contracts the <metal> orbitals, but can also change <electron> occupancy, ligand binding and electrostatic interactions. The <ligand field> and donor/acceptor properties of the surrounding <ligands in coordination chemistry> determine which <metal> orbitals remain available for direct bonding and how much density instead participates in <pi backbonding>.
Finally, the geometry can enforce or oppose the preferred separation. <Bridging ligands> impose bond-angle and bond-length constraints; bulky substituents produce <steric hindrance>; crystal packing can perturb the observed distance. \b[A <bond length> is evidence for bonding only in its electronic and structural context.] In particular, two bridges can hold <metals> close without proving a high direct <bond order>, while a sterically congested formal single bond can be unusually long.
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