Octa­chloro­di­rhen­ate

Other names: dirhenium(III) octachloride, octa­chlorido­dirhenate(III)

[Re₂Cl₈]^2-

49/50 rare

Typically found as the potassium salt K₂[Re₂Cl₈], this simple complex features a quadruple bond between the two rhenium centers, courtesy of a δ bond between the two 4d_xy orbitals in the rhenium centers. Although this species represents a fairly elegant example of metal-metal multiple bonding, it is far from the only example of the type; metal-metal bond orders as high as 5 (quintuple bonding) have been observed experimentally and sextuple bonding is theorized to exist in gas-phase dimolybdenum (Mo₂) and ditungsten (W₂) at temperatures above 4500 °C. Even higher-order bonding implicating f orbitals (φ bonding) has long been theorized, though despite decades of theorizing and computational modeling no diatomic bond orders above 6 are definitively known to exist.