Binuclear Rhodium(I) Carbonyl Carboxylate Complexes: DFT Study of Structural and Spectral Properties

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The geometry of binuclear rhodium(I) complexes, [Rh(μ-Cl)(CO)2]2 and [Rh(μ-RCOO)(CO)2]2 (R = H, CH3, CF3), was optimized by the DFT method; the vibrational spectra and the δ13C chemical shifts were calculated. The calculations were performed using the B3LYP hybrid functional and four basis sets. The calculations with the LanL2DZ basis set reflect all trends in the variation of the characteristics predicted using augmented basis sets and observed in the experiment. The Rh → CO electron density transfer along the π-bond and δ13C increase, while the bond orders, the intrinsic frequencies, and the force constants of the carbonyl groups in the carboxylate complexes decrease following a decrease in the substituent electronegativity in the series CF3 > H > CH3. It was found that the ratio of the v(CO) intensities in the IR spectra can be used to derive information on the dihedral angle between the Rh(CO)2 planes in the binuclear complexes. Calculations were carried out for a tetranuclear model system [Rh(HCOO)(CO)2]4, which can be treated as an elementary unit of the infinite chains of metal atoms in [Rh(μ-RCOO)(CO)2]2 stack crystals. A local minimum was found on the potential energy surface corresponding to the geometric structure of the crystal fragments.

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