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 . 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.
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. 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.
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]. An example of a ditungsten compound with a quadruple bond is di-tungsten tetra(hpp).
- 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
- Cotton, F. A.; Walton, R. A. “Multiple Bonds Between Metal Atoms” Oxford (Oxford): 1993. ISBN 0-19-855649-7.
- Girolami, G. S.; Rauchfuss, T. B. and Angelici, R. J., "Synthesis and Technique in Inorganic Chemistry," University Science Books: Mill Valley, CA, 1999.