Quintuple bond

  A quintuple bond in chemistry is an unusual type of chemical bond first reported in 2005 for a dichromium compound. Single bonds, double bonds, and triple bonds are commonplace in chemistry. Quadruple bonds are rarer but occur especially for Cr, Mo, W, and Re, e.g. [Mo₂Cl₈]^4- and [Re₂Cl₈]^2- .² In a quintuple bond, 10 electrons participate in bonding between the two metal centers, allocated as as σ²π⁴δ⁴. In some cases metal-metal bonding is facilitated by ligands that link the two metal centers and reduce the intermolecular distance. In contrast, the chromium dimer with quintuple bonding is stabilized by bulky 2,6-[(2,6-diisopropyl)phenyl]phenyl ligands or simply terphenyl ligand. The species is stable up to 200 °C.^[1][2] The chromium-chromium quintuple bond has been analyzed with multireference ab-initio and DFT methods.^[3] A 2007 theoretical study identified two global minima for quintuple bonded RMMR compounds: a trans-bent molecular geometry and surprisingly another trans-bent geometry with the R substituent in a bridging position.^[4]

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Also in 2005, a quintuple bond was postulated to exist in the hypothetical uranium molecule U₂ based on computational chemistry.^[5][6] Diuranium compounds are rare but do exist such as the U₂Cl₈^2- anion.

In 2007 the shortest ever metal to metal bond (1.8028 Å) was reported to exist also in a compound containing a quintuple chromium-chromium bond.^[7]

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