A DFT study on geometric preference of non-bridging form to bridging form in molybdenum complexes with phosphenium ligand

Density functional theory (DFT) calculations were conducted to elucidate why complexes bearing both phosphenium and phosphite ligands in a cis position do not take an OR-bridging form and why complexes bearing both silylene and alkoxysilyl ligands in a cis position prefer an OR-bridging form. Energy profiles, geometries, and electronic structures along the transformation pathways from the non-bridging to the bridging forms were analyzed for four phosphenium phosphite complexes, cis-[Mo(CO)4{P(NMeCH2)2}{P(NMeCH2)2(OMe)}]+ (1), CpMo(CO){P(NMeCH2)2}{P(NMeCH2)2(OMe)} (2), CpMo(CO){PMe2}{PMe2(OMe)} (3), cis-[Mo(CO)4(PR2){PR2(OMe)}]+ (R = Me, Et, n-Pr) (4), and a silylene alkoxysilyl complex CpMo(CO)2{(SiMe2)2(OMe)} (Si1). The DFT B3LYP/SBKJC(d) calculations for phosphenium phosphite complexes 1 and 2 revealed that there are two local minima (LM), both of which are non-bridging forms with similar energy levels, and one bridging form as a transition state (TS), which connects one non-bridging form and its mirror-image complex. These are consistent with the experimental results. In contrast, for silylene alkoxysilyl complex Si1, both bridging and non-bridging forms are LM. The former is more stable than the latter by 21.07 kcal/mol. The TS directly connects them in association with the rotation of the silyl ligand. The quite small activation energy less than 2 kcal/mol and the large energy difference between the two LM are consistent with the experimental results that only the bridging form has been reported to date. Phosphenium phosphite complexes 3 and 4 with alkyl substituents in place of amino substituents on the phosphenium P were also subjected to DFT calculations. The energy profile of 3 was found to be similar to those of 1 and 2. However, non-bridging and bridging forms were both LM for 4. The bridging form was estimated to be easily transformed to the non-bridging form, because the non-bridging form is more stable in energy and the activation energy from the bridging form is less than 1 kcal/mol for 4a. Charge accumulation in the bonding region, nuclear repulsion among the ligands, and the stability of E–O–E bond formation (E = P, Si) were considered to be decisive factors for the geometric feature.