Structure and internal rotation in quadruply bonded alpha-Mo2Cl4(P-P)(2) complexes: a density functional theory study of the cis-Mo2Cl4(PH3)(4) complex

The cis-Mo2Cl4(PH3)(4) complex is used as a model for the alpha-isomer of t he Mo2Cl4(P-P)(2) complexes where P-P is a diphosphine ligand. Density func tional theory calculations with the B3LYP functional are performed for the lowest singlet (delta(2)) and triplet ((3)delta delta*) states. The eclipse d C-2h conformation is found to be the most stable structure for both these electronic states, the singlet state being the electronic ground state. Th e singlet-tripler energy difference, which gives a measure of the delta-bon d energy, is equal to 10.9 kcal/mol (spin-projected broken-symmetry calcula tions). A rotational barrier around the Mo-Mo bond of about 27 kcal/mol is found for both electronic states, the singlet state remaining the electroni c ground state whatever the value of the internal rotational angle. (C) 199 9 Elsevier Science B.V. All rights reserved.