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 and are currently known only among the transition metals, especially for Cr, Mo, W, and Re, e.g. [Mo₂Cl₈]⁴⁻ and [Re₂Cl₈]²⁻. In a quintuple bond, ten electrons participate in bonding between the two metal centers, allocated as σ²π⁴δ⁴.

In some cases of high-order bonds between metal atoms, the metal-metal bonding is facilitated by ligands that link the two metal centers and reduce the interatomic distance. By contrast, the chromium dimer with quintuple bonding is stabilized by a bulky terphenyl (2,6-[(2,6-diisopropyl)phenyl]phenyl) ligands. 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] which were also used to elucidate the role of the terphenyl ligand, in which the flanking aryls were shown to interact very weakly with the chromium atoms, causing only a small weakening of the quintuple bond.^[4] 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.^[5]

In 2005, a quintuple bond was postulated to exist in the hypothetical uranium molecule U₂ based on computational chemistry.^[6][7] Diuranium compounds are rare, but do exist; for example, the U
₂Cl²⁻
₈ anion.

In 2007 the shortest-ever metal–metal bond (180.28 pm) was reported to exist also in a compound containing a quintuple chromium-chromium bond with diazadiene bridging ligands.^[8] Other metal–metal quintuple bond containing complexes that have been reported include quintuply bonded dichromium with [6-(2,4,6-triisopropylphenyl)pyridin-2-yl](2,4,6-trimethylphenyl)amine bridging ligands^[9] and a dichromium complex with amidinate bridging ligands.^[10]

Synthesis of quintuple bonds is usually achieved through reduction of a dimetal species using potassium graphite. This adds valence electrons to the metal centers, giving them the needed number of electrons to participate in quintuple bonding. Below is a figure of a typical quintuple bond synthesis.