NMR investigations of restricted rotations and metallotropic shifts in rhenium(I) tricarbonyl halide complexes of pyridyl carboxamide and thioamide ligands

A series of rhenium(I) tricarbonyl halide complexes of pyridyl mono- and di -carboxamides and -thioamides has been studied by dynamic NMR techniques. O xygen coordination to Re-I of the carboxamide ligand reduced the energy bar rier to C-N rotation by 2-13 kJ mol(-1) whereas sulfur-Re-I coordination of the thioamide ligands led to a reduction of 28-33 kJ mol(-1). In the pyrid yl dicarboxamide and -dithioamide metal chelate complexes metallotropic shi fts occur between the three donor atoms, O, N, O or S, N, S, energies [Delt a G(double dagger) (298.15 K)] being in the range 78-89 kJ mol(-1); these h igh values arise from the strong electron donating properties of the -NR2 ( R = Me, Et) groups.