A metal-metal bond refers to the interaction between metal atoms in a solid or liquid state. These bonds are primarily characterized by the sharing of delocalized electrons in what is often termed a "metallic bond." In a metallic bond, metal atoms collectively pool their valence electrons, which form a "sea of electrons" that are free to move throughout the metal lattice.
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A metal-metal bond joins two metal centers directly through overlap of their atomic orbitals. A single bond contributes one electron to the local count at each metal in conventional electron counting. Bridging ligands can accompany direct bonding, but a short bridged separation alone does not establish its bond order.