Steric versus electronic effects in six-co-ordinate d(o) cis-bis(imido) molybdenum complexes

The previously reported molybdenum(VI) cis-bis(imido) complex [Mo(Nmes)(2)C l-2(PMe3)(2)] 2 (mes=C6H2Me3-2,4,6) displayed NMR properties consistent wit h a cis-chloride,trans-PMe3 configuration at the metal centre. LSDA density functional calculations on the cis-bis(imido) models [Mo(NR)(2)Cl-2(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-chl oride,trans-PR3'; trans-chloride,cis-PR3' and cis-chloride,cis-PR3') of suc h species were examined. The actual stereochemistry of 2 was properly repro duced by the calculations as the most stable isomer, only when no simplific ation of PMe3 ligand by PH3 was considered in the model. The use of PH3 yie lds the cis-chloride,trans-PH3 configuration as the most unstable isomer. T he inversion in the trend of relative stabilities of the three isomers is d iscussed on the basis of steric considerations. The tendency of pi-donor ch loride ligands to occupy cis positions with respect to the d(0) bis(imido) moiety was overcome by the steric incompatibility of two cis-PMe3 ligands. The molecular structure of [Mo(Nmes)(2)Cl-2(dme)] 1 was determined by an X- ray study and the reaction of 1 with PMe3 to yield 2 revised.