Quadruple bond

[1] Stable quadruple bonds are most common among the transition metals in the middle of the d-block, such as rhenium, tungsten, technetium, molybdenum and chromium.

[2] Chromium(II) acetate, Cr2(μ-O2CCH3)4(H2O)2, was the first chemical compound containing a quadruple bond to be synthesized.

[3] The first crystallographic study of a compound with a quadruple bond was provided by Soviet chemists for salts of Re2Cl2−8.

[6] Many other compounds with quadruple bonds between transition metal atoms have been described, often by Cotton and his coworkers.

A 2012 paper by S. Shaik et al. suggests that a quadruple bond exists in dicarbon,[8] but this is disputed.

The structure of chromium(II) acetate contains a quadruple Cr–Cr bond.
The octachlorodirhenate(III) anion, [Re 2 Cl 8 ] 2− , which features a quadruple Re–Re bond
Formation of a delta bond by the overlap of two d orbitals