Metal-metal bond

In inorganic chemistry, metal-metal bonds describe attractive interactions between metal centers. The simplest examples are found in bimetallic complexes. Metal-metal bonds can be "supported", i.e. be accompanied by one or more bridging ligands, or "unsupported". They can also vary according to bond order. The topic of metal-metal bonding is usually discussed within the framework of coordination chemistry,[1] but the topic is related to extended metallic bonding, which describes interactions between metals in extended solids such as bulk metals and metal subhalides.[2]

dimanganese decacarbonyl.

Unsupported metal-metal bonds

The premier example of a complex with an unsupported metal-metal bond is dimanganese decacarbonyl, Mn2(CO)10. Many metal clusters contain several unsupported M-M bonds. Some examples are M3(CO)12 (M = Ru, Os) and Ir4(CO)12.

A subclass of unsupported metal-metal bonded arrays are linear chain compounds. In such cases the M-M bonding is weak as signaled by longer M-M bonds and the tendency of such compounds to dissociate in solution.

Supported metal-metal bonds

An early example of a supported metal-metal bond is cyclopentadienyliron dicarbonyl dimer, [(C5H5)Fe(CO)2]2. In the predominant isomers of this complex, the two Fe centers are joined not only by an Fe-Fe bond, but also by bridging CO ligands. The related cyclopentadienylruthenium dicarbonyl dimer features an unsupported Ru-Ru bond. Many metal clusters contain several supported M-M bonds. Some examples are Fe3(CO)12 and Co4(CO)12.

Multiple metal-metal bonds
  • Nb2Cl6(SMe2)3, featuring a metal-metal double bond.
  • structure of W2(OBu-t)6
  • Chromium(II) acetate, featuring a metal-metal quadruple bond.
  • [Ru2(OAc)4(H2O)2]+, where the metal-metal bond order is 2.5.

In addition to M-M single bonds, metal pairs can be linked by double, triple, quadruple, and in a few cases, quintuple bonds.[3] Isolable complexes with multiple bonds are most common among the transition metals in the middle of the d-block, such as rhenium, tungsten, technetium, molybdenum and chromium. Typical the coligands are π-donors, not π-acceptors.[4] Well studied examples are the tetraacetates, such as dimolybdenum tetraacetate (quadruple bond) and dirhodium tetraacetate (single bond). Mixed-valence druthenium tetraacetates have fractional M-M bond orders, i.e., 2.5 for [Ru2(OAc)4(H2O)2]+.[5]

The complexes Nb2X6(SR2)3 adopt a face-sharing bioctahedral structures (X = Cl, Br; SR2 = thioether). As dimers of Nb(III), they feature double metal-metal bonds, the maximum possible for a pair of metals with d2 configuration.[6] Hexa(tert-butoxy)ditungsten(III) is a well studied example of a complex with a metal-metal triple bond.[7]

References
  1. Berry, John F.; Lu, Connie C. (2017). "Metal–Metal Bonds: From Fundamentals to Applications". Inorganic Chemistry. 56 (14): 7577–7581. doi:10.1021/acs.inorgchem.7b01330. PMID 28715854.
  2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  3. Radius, U.; Breher, F. (2006). "'To Boldly Pass the Metal–Metal Quadruple Bond". Angew. Chem. Int. Ed. 45 (19): 3006–3010. doi:10.1002/anie.200504322. PMID 16642513.
  4. Cotton, F. A.; Walton, R. A. (1993). Multiple Bonds Between Metal Atoms. Oxford: Oxford University Press. ISBN 0-19-855649-7.
  5. Aquino, Manuel A.S. (1998). "Diruthenium and diosmium tetracarboxylates: Synthesis, physical properties and applications". Coordination Chemistry Reviews. 170: 141–202. doi:10.1016/S0010-8545(97)00079-9.
  6. Kakeya, Masaki; Fujihara, Takashi; Nagasawa, Akira (2006). "Di-μ-chloro-μ-(dimethyl sulfide)-bis[dichloro(dimethyl sulfide)niobium(III)]". Acta Crystallographica Section E. 62 (3): m553–m554. doi:10.1107/S1600536806005149.
  7. Broderick, Erin M.; Browne, Samuel C.; Johnson, Marc J. A. (2014). "Dimolybdenum and Ditungsten Hexa(Alkoxides)". Inorganic Syntheses. 36: 95–102. doi:10.1002/9781118744994.ch18.
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