Quadruple bond

A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds.[1] Stable quadruple bonds are most common among the middle members transition metal elements such rhenium, tungsten, molybdenum and chromium. Typically the ligands that support quadruple bonds are π-donors, not π-acceptors.
The octachlorodirhenate(III) anion, [Re₂Cl₈]²⁻, which features a quadruple Re-Re bond

Chromium(II) acetate, Cr₂(μ-O₂CMe)₄(H₂O)₂, was the first chemical compound containing a quadruple bond to be synthesized. It was described in 1844 by E. Peligot, although its distinctive bonding was not recognized for more than a century.[2] The quadruple bond was first characterized in potassium octachlorodirhenate(III) or K₂[Re₂Cl₈]·2H₂O by F. A. Cotton in 1964. The rhenium-rhenium bond length in this compound is only 224 pm. In the terminology of molecular orbital theory, the bonding is described as σ²π⁴δ² with one sigma bond, two π bonds and one delta bond.

Many other compounds with quadruple bonds have been described, often by Cotton and his coworkers. Isoelectronic with the dirhenium compound is the salt K₄[Mo₂Cl₈] (potassium octachlorodimolybdate).[3] An example of a ditungsten compound with a quadruple bond is di-tungsten tetra(hpp).
The structure of the hydrated diacetates of molybdenum(II) and chromium(II)

References

1. ^ To Boldly Pass the Metal-Metal Quadruple Bond Radius, U.; Breher, F. Angewandte Chemie International Edition 2006, Volume 45, Issue 19 , Pages 3006 - 3010 Abstract
2. ^ Cotton, F. A.; Walton, R. A. "Multiple Bonds Between Metal Atoms" Oxford (Oxford): 1993. ISBN 0-19-855649-7.
3. ^ Girolami, G. S.; Rauchfuss, T. B. and Angelici, R. J., "Synthesis and Technique in Inorganic Chemistry", University Science Books: Mill Valley, CA, 1999.