Quadruple bond

A quadruple bond is a type of chemical bond between two atoms involving 8 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.

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  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 2.24 Angstrom. In the terminology of molecular orbital theory, the bonding is described as σ²π⁴δ² with one sigma bond, two π bonds and one delta bond. The strength of the delta bond is a matter of debate. The bond order for the [Re₂Cl₈]²⁻ is calculated to be 3.2, not 4.^{[citation needed]}

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].^[3] An example of a ditungsten compound with a quadruple bond is di-tungsten tetra(hpp).

See also

• Covalent bond:Bond order
• Quintuple bond

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.