SCALAR AND DIPOLAR CONTRIBUTIONS OF RHENIUM TO THE RELAXATION PROPERTIES OF COORDINATED LIGANDS

The longitudinal relaxation times (T1) of the hydridic ligands and of P-31 in [HRe(CO)4PPh3] (1), [HRe(CO)5] (2), and [Re2-(mu-H)2(CO)8] (3) have been measured as a function of the temperature and of the applied magnetic field. In 1 the relaxation of P-31 is dominated by the scalar interactions with the quadrupolar isotopes of rhenium up to 7 T. The field dependence of the relaxation rate of phosphorus has allowed the calculation of the coupling constant 1J(Re-P) (1260 +/- 20 Hz) and of the Re relaxation time (22 +/- 1 ns). For the hydridic ligand, dipolar interactions with rhenium account for more than 60% of the observed relaxation rate. 1J(H-Re) for 1 has been estimated as 440 Hz. The relaxation of the classical hydrides in 2 and 3 is completely determined by dipolar interactions with rhenium. 1J(H-Re) for 2 has been determined to be 385 Hz. The length of the Re-H bond (189 +/- 1 pm) in 3 has been calculated from the fitting of the temperature dependence of H-1 relaxation. Anisotropic reorientation was observed for compound 1. The relevance of the H-Re dipolar interaction in classical rhenium polyhydrides is discussed.