Structural preferences of quadruply bonded bimetallic complexes. A DFT study of the chelated (α) and bridged (β) isomers in Mo2Cl4(H2P(CH2)nPH2)2 (n=1, 2)

The structure of the quadruply bonded Mo2Cl4(H2P(CH2)nPH2)2 (n=1, 2) bimetallic complexes, with two chelating (α isomer) or two bridging (β isomer) diphosphine ligands, is studied by means of DFT calculations with the B3LYP functional. For each complex (1-α, 1-β and 2-α, 2-β, for n=1 and 2, respectively), full geometry optimization is performed for both the lowest singlet (δ2) and triplet (3δδ*) states. The chelated α isomers (1-α and 2-α) adopt an essentially eclipsed conformation in both the singlet and the triplet states. The 1-β complex is found to be staggered in the triplet state. Three structures are optimized for the 2-β isomer with a chair (2-βc), a twist (2-βt) or a boat (2-βb) conformation of the MoPCCPMo six-membered rings. The 2-βc and 2-βt complexes adopt a twisted conformation in both the singlet and the triplet states while the 2-βb complex is eclipsed in the singlet state. A singlet ground state is found for all these complexes, the β isomer being more stable than the α isomer by 17.1 and 7.0 kcal mol−1 for n=1 and n=2, respectively. These geometrical and energetic results are discussed with respect to available experimental data.