metal-metal bonding
We usually picture a metal compound as a single metal atom surrounded by ligands, with the metal at the center like a lone hub. But sometimes two (or more) metal atoms in a compound bond directly to each other, sharing electrons between them just as two carbon atoms do in an organic molecule. This direct metal-to-metal handshake is a real chemical bond, and recognizing it changes how we count and understand the compound.
What makes metal-metal bonds special is that the d orbitals can overlap in extra ways that p orbitals cannot. Two metals can form a single (sigma) bond by overlapping orbitals head-on, two pi bonds by overlapping sideways, and remarkably a fourth kind, the delta bond, by overlapping two parallel d orbitals face-to-face. Stack all four together and you get a quadruple bond — something impossible for main-group elements. The classic case is the dirhenium ion [Re2Cl8]2-, where two rhenium atoms are held by a quadruple bond so strong that the eight chloride ligands are forced into an eclipsed (lined-up) arrangement, which only the delta bond can explain. Metal-metal bonds are far more common among the heavier 4d and 5d metals, whose larger, more diffuse d orbitals overlap better, and they are favored in lower oxidation states where the metals still hold enough d electrons to share.
Metal-metal bonding is the gateway to metal clusters — compounds with whole frameworks of bonded metal atoms — and it bridges the gap between isolated molecules and bulk metal, where every atom is bonded to its neighbors. It shows up in industrial catalysts, in the structures of metal carbonyls, and in the chemistry of the platinum-group metals. Spotting a metal-metal bond is essential for getting the electron count right, since the bonding electrons are shared between the metals rather than belonging to ligands.
In [Re2Cl8]2- two rhenium atoms are joined by a quadruple bond: one sigma, two pi, and one delta. The delta bond can only form if the two ReCl4 halves line up directly (eclipsed) rather than staggering to avoid each other. The fact that the molecule chooses the seemingly crowded eclipsed shape is the experimental fingerprint of the delta bond.
The quadruple Re-Re bond (sigma + 2 pi + delta) forces the eclipsed shape of [Re2Cl8]2-.
Metal-metal bonds are mostly a feature of lower oxidation states and the heavier transition metals; first-row metals form them less readily because their compact 3d orbitals overlap poorly. Do not assume two metals close together in a structure are bonded — bridging ligands can hold them near without any direct M-M bond.