PHOTOELECTRON-SPECTRA AND MOLECULAR-ORBITAL CALCULATIONS ON BIS(CYCLOPENTADIENYLDICARBONYLCHROMIUM, MOLYBDENUM, AND TUNGSTEN) - NATURE OF THE BONDING OF LINEAR SEMIBRIDGING CARBONYLS
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The gas-phase ultraviolet photoelectron spectra are reported for the compounds [(5-C5H5)M(CO)2]2, where M = Cr, Mo, W. The spectra are compared to Fenske-Hall molecular orbital calculations on the chromium and molybdenum species. The 15-electron fragment, (5-C5H5)M(CO)2, requires three additional electrons to satisfy the 18-electron rule. The fragments dimerize and the dimer can be described as a metal-to-metal triple bond with linear semibridging carbonyls. The spectra for all the dimers are similar in general appearance. The spectrum of the chromium species, however, has ionizations from the 15bu and 14ag orbitals that appear as distinct peaks at lower ionization energy than they do in the spectra of the molybdenum and tungsten species. These changes are a result of the nonlinear Cp-M-M framework in the chromium dimers. The calculations suggest that the linear semibridging carbonyls are acceptors, not donors. They bend over the MM bond in order to accept electrons from the metal-metal bonds, but they remain linear to avoid destroying this bond. Thus, they join with the two metals to form multicenter, two-electron bonds. 1984, American Chemical Society. All rights reserved.
