DFT calculations of NMR JC-H coupling constants:: An additional tool to characterize the α-agostic interaction in high oxidation state M-alkylidene complexes (M = Re, Mo and Ta)

NMR spin-spin J(C-H) coupling constants have been calculated for the alpha-alkylidene C-H bond in several high oxidation state do transition metal-alkylidene complexes (M = Re, Mo, Ta). The DFT(B3PW91) calculations have been carried out using the GIAO method for the isolated molecules, with no solvent effect or thermal corrections. The experimental systems have been modeled either by replacing all large size ligands by ligands of smaller size with similar electronic properties or by calculating the full chemical entities. It is shown that the calculations are able to reproduce well the trends in the J(C-H) values over a range of 75-150 Hz when the IGLO basis sets are used. In addition, the calculated J(C-H) values differ from the experimental ones by about 10% in the case of weak alpha-C-H agostic interactions (pseudotetrahedral Re and Mo complexes) and by 15% in the case of strong alpha-C-H agostic interactions (pseudooctahedral Ta complexes). This shows that the electronic environment of the alkylidene C-H bonds, associated with the alpha-agostic interaction, is properly evaluated at this level of calculations for transition metal complexes of rather large size. (c) 2005 Elsevier Ltd. All rights reserved.