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Structure and bonding

The octachlorodirhenate(III) anion, [Re2Cl8]2−, which features a quadruple Re–Re bond

Formation of a delta bond by the overlap of two d orbitals The [Re2Cl8]2− ion adopts an eclipsed conformation as shown at left. The delta bonding orbital is then formed by overlap of the d orbitals on each rhenium atom, which are perpendicular to the Re–Re axis and lie in between the Re–Cl bonds. The d orbitals directed along the Re–Cl bonds are stabilized by interaction with chlorine ligand orbitals and do not contribute to Re–Re bonding.[5] In contrast, the [Os2Cl8]2− ion with two more electrons (σ2π4δ2δ*2) has an Os–Os triple bond and a staggered geometry.[5]

Many other compounds with quadruple bonds between transition metal atoms have been described, often by Cotton and his coworkers. Isoelectronic with the dirhenium compound is the salt K4[Mo2Cl8] (potassium octachlorodimolybdate).[6] An example of a ditungsten compound with a quadruple bond is ditungsten tetra(hpp).

Quadruple bonds between atoms of main group elements are unknown. Molecular orbital theory shows that there are two sets of paired electrons in the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate π-bonding set of orbitals. This adds up to give a bond order of 2, meaning that there exists a double bond between the two carbons in a dicarbon (C2) molecule. The molecular orbital diagram of diatomic carbon would show that there are two pi bonds and no sigma bonds. However, a recent paper by S. Shaik et al. has suggested that a quadruple bond exists in diatomic carbon.[7]