Nature of Bonding in Complexes Containing 'Supershort' Metal-Metal Bonds. Raman and Theoretical Study of M_2(dmp)_4 [M = Cr (Natural Abundance Cr, ~(50)Cr, and ~(54)Cr) and Mo; dmp = 2,6-Dimethoxyphenyl]

摘要

We report an investigation of complexes of the type M_2(dmp)_4 (M = Mo, Cr; dmp = 2,6-dimethoxyphenyl) using resonance Raman (RR) spectroscopy, Cr isotopic substitution, and density functional theory (DFT) calculations. Assignment of the Mo-Mo stretching vibration in the Mo_2 species is straightforward, as evidenced by a single resonance-enhanced band at 424 cm~(-1), consistent with an essentially unmixed metal-metal stretch, and overtones of this vibration. On the other hand, the Cr_2 congener has no obvious metal-metal stretching mode near 650-700 cm~(-1), where empirical predictions based on the Cr-Cr distance as well as DFT calculations suggest that this vibration should appear if unmixed. Instead, three bands are observed at 345, 363, and 387 cm~(-1) that (a) have relative RR intensities that are sensitive to the Raman excitation frequency, (b) exhibit overtones and combinations in the RR spectra, and (c) shift in frequency upon isotopic substitution (~(50)Cr and ~(54)Cr). DFT calculations are used to model the vibrational data for the Mo_2 and Cr_2 systems. Both the DFT results and empirical predictions are in good agreement with experimental observations in the Mo_2 complex, but both, while mutually consistent, differ radically from experiment in the Cr_2 complex. Our experimental and theoretical results, especially the Cr isotope shifts, clearly demonstrate that the potential energy of the Cr-Cr stretching coordinate is distributed among several normal modes having both Cr-Cr and Cr-ligand character. The general significance of these results in interpreting spectroscopic observations in terms of the nature of metal-metal multiple bonding is discussed.