Quintuple bond

The structure of [CrC6H3-2,6-(C6H3-2,6-(CHMe2)2)2]2

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.

Cr–Cr quintuple bond synthesis
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  2. ^ Nguyen, Tailuan; Sutton, Andrew D.; Brynda, Marcin; Fettinger, James C.; Long, Gary J.; Power, Philip P. (2005). "Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers". Science. 310 (5749): 844–847. Bibcode:2005Sci...310..844N. doi:10.1126/science.1116789. PMID 16179432. S2CID 42853922.
  3. ^ Brynda, Marcin; Gagliardi, Laura; Widmark, Per-Olof; Power, Philip P.; Roos, Björn O. (2006). "Quantum Chemical Study of the Quintuple Bond between Two Chromium Centers in [PhCrCrPh]: trans-Bent versus Linear Geometry". Angew. Chem. Int. Ed. 45 (23): 3804–3807. doi:10.1002/anie.200600110. PMID 16671122.Open access icon
  4. ^ La Macchia, Giovanni; Gagliardi, Laura; Power, Philip P.; Brynda, Marcin (2008). "Large Differences in Secondary Metal−Arene Interactions in the Transition-Metal Dimers ArMMAr (Ar = Terphenyl; M = Cr, Fe, or Co): Implications for Cr−Cr Quintuple Bonding". J. Am. Chem. Soc. 130 (15): 5104–5114. doi:10.1021/ja0771890. PMID 18335988. S2CID 207046428.
  5. ^ Merino, Gabriel; Donald, Kelling J.; D'Acchioli, Jason S.; Hoffmann, Roald (2007). "The Many Ways To Have a Quintuple Bond". J. Am. Chem. Soc. 129 (49): 15295–15302. doi:10.1021/ja075454b. PMID 18004851. S2CID 18838267.
  6. ^ Gagliardi, Laura; Roos, Björn O. (24 February 2005). "Quantum chemical calculations show that the uranium molecule U2 has a quintuple bond". Nature. 433 (7028): 848–851. Bibcode:2005Natur.433..848G. doi:10.1038/nature03249. PMID 15729337. S2CID 421380.
  7. ^ Dumé, Belle (23 February 2005). "New look for chemical bonds". PhysicsWeb.
  8. ^ Kreisel, Kevin A.; Yap, Glenn P. A.; Dmitrenko, Olga; Landis, Clark R.; Theopold, Klaus H. (2007). "The Shortest Metal–Metal Bond Yet: Molecular and Electronic Structure of a Dinuclear Chromium Diazadiene Complex". J. Am. Chem. Soc. (Communication). 129 (46): 14162–14163. doi:10.1021/ja076356t. PMID 17967028.
  9. ^ Noor, Awal; Wagner, Frank R.; Kempe, Rhett (2008). "Metal–Metal Distances at the Limit: A Coordination Compound with an Ultrashort Chromium–Chromium Bond". Angew. Chem. Int. Ed. 47 (38): 7246–7249. doi:10.1002/anie.200801160. PMID 18698657. S2CID 30480347.
  10. ^ Tsai, Yi-Chou; Hsu, Chia-Wei; Yu, Jen-Shiang K.; Lee, Gene-Hsiang; Wang, Yu; Kuo, Ting-Shen (2008). "Remarkably Short Metal–Metal Bonds: A Lantern-Type Quintuply Bonded Dichromium(I) Complex". Angew. Chem. Int. Ed. 47 (38): 7250–7253. doi:10.1002/anie.200801286. PMID 18683844. S2CID 5510753.