Linear Semibridging Carbonyls. 6. Structure and Bonding in the Dimers of 17-Electron Tantalum Hexacarbonyl and Tetracarbonyl Diphosphine

The geometric and electronic structures of the 17-electron, metal-centered radicals Ta(CO)_6 and Ta(CO)_4(PH_2CH_2CH_2PH_2) and the corresponding dimers were studied by density functional theory. For the monomer, a nearly octahedral structure was determined and calculated to have frequencies that are in good agreement with those observed in the experimental work. Additional calculations led to the determination of a [Ta(CO)_6]_2 dimer that was energetically bound with respect to the monomeric species. Inclusion of the bidentate phosphine ligands in various relative orientations resulted in several energetically competitive chelated dimers. In all of the stable dimers, linear semibridging carbonyls are supporting a weak, delocalized Ta−Ta interaction