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 are currently known only among the transition metals, especially for Cr, Mo, W, and Re, e.g. 4− and 2−. In a quintuple bond, ten electrons participate in bonding between the two metal centers, allocated as σ2π4δ4. 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 are currently known only among the transition metals, especially for Cr, Mo, W, and Re, e.g. 4− and 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-phenyl) ligands. The species is stable up to 200 °C. The chromium–chromium quintuple bond has been analyzed with multireference ab initio and DFT methods, 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. 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. In 2005, a quintuple bond was postulated to exist in the hypothetical uranium molecule U2 based on computational chemistry. Diuranium compounds are rare, but do exist; for example, the U2Cl2−8 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. Other metal–metal quintuple bond containing complexes that have been reported include quintuply bonded dichromium with (2,4,6-trimethylphenyl)amine bridging ligands and a dichromium complex with amidinate bridging ligands. 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. In 2009 a dimolybdenum compound with a quintuple bond and two diamido bridging ligands was reported with a Mo–Mo bond length of 202 pm. The compound was synthesised starting from potassium octachlorodimolybdate (which already contains a Mo2 quadruple bond) and a lithium amidinate, followed by reduction with potassium graphite: As stated above metal-metal quintuple bonds have a σ2π4δ4 configuration. Among the five bonds present between the metal centers, one is a sigma bond, two are pi bonds, and two are delta bonds. The σ-bond is the result of mixing between the dz2 orbital on each metal center. The first π-bond comes from mixing of the dyz orbitals from each metal while the other π-bond comes from the dxz orbitals on each metal mixing. Finally the δ-bonds come from mixing of the dxy orbitals as well as mixing between the dx2−y2 orbitals from each metal. Molecular orbital calculations have elucidated the relative energies of the orbitals created by these bonding interactions. As shown in the figure below, the lowest energy orbitals are the π bonding orbitals followed by the σ bonding orbital. The next highest are the δ bonding orbitals which represent the HOMO. Because the 10 valence electrons of the metals are used to fill these first 5 orbitals, the next highest orbital becomes the LUMO which is the δ* antibonding orbital. Though the π and δ orbitals are represented as being degenerate, they in fact are not. This is because the model shown here is a simplification and that hybridization of s, p, and d orbitals is believed to take place, causing a change in the orbital energy levels. Quintuple bond lengths are heavily dependent on the ligands bound to the metal centers. Nearly all complexes containing a metal–metal quintuple bond have bidentate bridging ligands, and even those that do not, such as the terphenyl complex mentioned earlier, have some bridging characteristic to it through metal–ipso-carbon interactions.