Metal−Metal Quintuple and Sextuple Bonding in Bent Dimetallocenes of the Third Row Transition Metals
ABSTRACT Theoretical studies on the dimetallocenes Cp2M2 (M = Os, Re, W, Ta) predict bent structures with short metal−metal distances suggesting high-order metal−metal multiple bonds. Analysis of the frontier bonding molecular orbitals indicates a formal Os−Os quintuple bond (σ + 2π + 2δ) in singlet Cp2Os2 and a formal Re−Re sextuple bond (2σ + 2π + 2δ) in singlet Cp2Re2, thereby giving the metals in both molecules the favored 18-electron metal configurations. Predicted low-energy triplet structures for Cp2M2 (M = Os, Re) have formal quintuple bonds but with only two δ one-electron “half” bonds (M = Os) or a single δ two-electron bond (M = Re) and a second σ component derived from overlap of the d(z2) orbitals. A quintuple bond similar to that found in triplet Cp2Re2 is found in singlet Cp2W2, giving both tungsten atoms a 16-electron configuration. The formal Ta−Ta quadruple bond in the lowest energy singlet Cp2Ta2 structure is different from that in the original Re2Cl82− in that it is a 2σ + 2π bond with no δ components but only σ and π components.
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ABSTRACT: Triple bonding is conventionally considered to be the limit for multiply bonded main group elements, despite higher metal-metal bond orders being frequently observed for transition metals and lanthanides/actinides. Here, using high-level theoretical methods, we show that C(2) and its isoelectronic molecules CN(+), BN and CB(-) (each having eight valence electrons) are bound by a quadruple bond. The bonding comprises not only one σ- and two π-bonds, but also one weak 'inverted' bond, which can be characterized by the interaction of electrons in two outwardly pointing sp hybrid orbitals. A simple way of assessing the energy of the fourth bond is proposed and is found to be ~12-17 kcal mol(-1) for the isoelectronic species studied, and thus stronger than a hydrogen bond. In contrast, the analogues of C(2) that contain higher-row elements, such as Si(2) and Ge(2), exhibit only double bonding.Nature Chemistry 01/2012; 4(3):195-200. · 21.76 Impact Factor