Syntheses, Structures, and DFT Calculations of Phosphenium Phosphite Complexes of Molybdenum: Preference of Nonbridging Form to Bridging Form of a Donor Group

Molybdenum carbonyl complexes with two and three diamino-substituted phosphites, cis-[Mo(CO)4{P(NMeCH2)2(OMe)}2] (1a) and fac-[Mo(CO)3{P(NMeCH2)2(OMe)}3] (2a), react with TMSOTf to give
the corresponding cationic phosphenium phosphite complexes, cis-[Mo(CO)4{P(NMeCH2)2(OMe)}{P(NMeCH2)2}](OTf) (1b) and fac-[Mo(CO)3{P(NMeCH2)2(OMe)}2{P(NMeCH2)2}](OTf) (2b), respectively, by single OMe- abstraction from the coordinating phosphite. The X-ray structure analyses and
the NMR spectra of the products showed that the original configuration around the Mo is retained with
the OMe group(s) on the remaining phosphite ligand(s) free from the interaction with the phosphenium
phosphorus. A related phosphite complex, CpMo(CO)(I){P(NMeCH2)2(OMe)}2 (3), was converted in
the reaction with NaK2.8 into a neutral phosphenium complex, CpMo(CO){P(NMeCH2)2(OMe)}{P(NMeCH2)2} (4), in which no such significant MeO···P (phosphenium) bridging interaction was also
observed. DFT calculations have been incorporated to study the preference of the nonbridging form for
phosphenium phosphite complexes, together with the preference of the bridging form for the analogous
silylene alkoxysilyl complexes, and revealed that the bridging requires larger geometrical changes for
phosphenium phosphite complexes than for silylene alkoxysilyl complexes.