CHEMISTRY: Bend Origins

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Science  19 May 2006:
Vol. 312, Issue 5776, pp. 975b
DOI: 10.1126/science.312.5776.975b

Chemical paradigms for multiple bonding were recently challenged by the synthesis of a chromium dimer that appeared to be held together by the interaction of 10 electrons between the Cr centers (see Nguyen et al., Reports, 4 November 2005 p. 844). Before this discovery, isolable compounds were limited to bonding motifs in which eight or fewer electrons were shared between any two atoms. Orbital conformations in a quintuple bonding framework were largely expected to induce a linear geometry, but the bulky triaryl ligands capping the Cr centers adopted a bent, mutually trans configuration.

Brynda et al. have analyzed this geometrical conundrum using high-level quantum-mechanical calculations incorporating multiconfigurational perturbation theory. For a model compound with phenyl groups in place of the triaryl ligands, the linear conformer was energetically favored over the bent form by only 1 kcal/mol. Orbital occupancy analyses were consistent with participation of all 10 electrons in both conformers, though with repulsive antibonding contributions lowering effective bond orders to 3.69 and 3.52 for the linear and bent forms, respectively. — JSY

Angew. Chem. Int. Ed. 45, 10.1002/anie.200600110 (2006).

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