Structure and internal rotation in quadruply bonded α-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 α-isomer of the Mo2Cl4(P-P)2 complexes where P-P is a diphosphine ligand. Density functional theory calculations with the B3LYP functional are performed for the lowest singlet (δ2) and triplet (3δδ*) states. The eclipsed C2h conformation is found to be the most stable structure for both these electronic states, the singlet state being the electronic ground state. The singlet-triplet energy difference, which gives a measure of the δ-bond energy, is equal to 10.9 kcal/mol (spin-projected broken-symmetry calculations). A rotational barrier around the Mo-Mo bond of about 27 kcal/mol is found for both electronic states, the singlet state remaining the electronic ground state whatever the value of the internal rotational angle.