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.