| Abstract
| - Recent crystallographic data unambiguously demonstrate that neither Ar‘GeGeAr‘ nor Ar‘CrCrAr‘molecules adopt the expected linear (VSEPR-like) geometries. Does the adoption of trans-bent geometriesindicate that Ar‘MMAr‘ molecules are not “maximally bonded” (i.e., bond order of three for M = Ge and fivefor M = Cr)? We employ theoretical hybrid density functional (B3LYP/6-311++G**) computations and naturalbond orbital-based analysis to quantify molecular bond orders and to elucidate the electronic origin ofsuch unintuitive structures. Resonance structures based on quintuple M−M bonding dominate for thetransition metal compounds, especially for molybdenum and tungsten. For the main group, M−M bondingconsists of three shared electron pairs, except for M = Pb. For both d- and p-block compounds, the M−Mbond orders are reflected in torsional barriers, bond−antibond splittings, and heats of hydrogenation in aqualitatively intuitive way. Trans-bent structures arise primarily from hybridization tendencies that yield thestrongest σ-bonds. For transition metals, the strong tendency toward sd-hybridization in making covalentbonds naturally results in bent ligand arrangements about the metal. In the p-block, hybridization tendenciesfavor high p-character, with increasing avidity as one moves down the Group 14 column, and nonlinearstructures result. In both the p-block and the d-block, bonding schemes have easily identifiable Lewis-likecharacter but adopt somewhat unconventional orbital interactions. For more common metal−metal multiplybonded compounds such as [Re2Cl8]2-, the core Lewis-like fragment [Re2Cl4]2+ is modified by fourhypervalent three-center/four-electron additions.
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