Steric versus electronic effects in six-co-ordinate d0cis-bis(imido) molybdenum complexes

Abstract

The previously reported molybdenum(VI) cis-bis(imido) complex [Mo(Nmes)₂Cl₂(PMe₃)₂] 2 (mes = C₆H₂Me₃-2,4,6) displayed NMR properties consistent with a cis-chloride,trans-PMe₃ configuration at the metal centre. LSDA density functional calculations on the cis-bis(imido) models [Mo(NR)₂Cl₂(PR′₃)₂] (R = H, Me or Ph for R′ = H and R = H for R′ = Me) were performed in order to establish the reason why 2 does not adopt the expected trans-chloride,cis-PMe₃ geometry. The relative stabilities of three different isomers (cis-chloride,trans-PR′₃; trans-chloride,cis-PR′₃ and cis-chloride,cis-PR′₃) of such species were examined. The actual stereochemistry of 2 was properly reproduced by the calculations as the most stable isomer, only when no simplification of PMe₃ ligand by PH₃ was considered in the model. The use of PH₃ yields the cis-chloride,trans-PH₃ configuration as the most unstable isomer. The inversion in the trend of relative stabilities of the three isomers is discussed on the basis of steric considerations. The tendency of π-donor chloride ligands to occupy cis positions with respect to the d⁰ bis(imido) moiety was overcome by the steric incompatibility of two cis-PMe₃ ligands. The molecular structure of [Mo(Nmes)₂Cl₂(dme)] 1 was determined by an X-ray study and the reaction of 1 with PMe₃ to yield 2 revised.