P-31 CHEMICAL-SHIFT TENSORS OF PHOSPHINIDENE LIGANDS IN RUTHENIUM CARBONYL CLUSTER COMPOUNDS - A P-31 SINGLE-CRYSTAL AND CP/MAS-NMR STUDY

Phosphorus chemical shift tensors of the phosphimidene moiety, formally RP(2-), in several solid ruthenium carbonyl clusters have been characterized using slow magic-angle spinning P-31 NMR techniques. Typically, the phosphorus nucleus in these systems is highly deshielded relative to the standard reference, 85% H3PO4(aq), with isotropic chemical shifts in the range of 200-550 ppm. For one representative cluster, nido-Ru-4(CO)(13)(mu(3)-PPh), 2, P-31 NMR studies of a single crystal yielded the principal components df the phosphorus chemical;shift (CS) tenser (delta(11) = 889, delta(22) = 294, and delta(33) = 60 ppm) as well as the orientation of the CS tenser. pronounced when the applied-magnetic field, B-0, is oriented approximately parallel to the P-C bond of the PPh moiety, while the greatest shielding occurs when B-0 is approximately perpendicular to the P-C bond and the open face of the nido-Ru-4 cluster. The orientation of the phosphorus chemical shift tensor is discussed in the Context of Ramsey's theory of nuclear magnetic shielding and the results of extended Huckel molecular orbital calculations. With the P-C bond axis defined as the x-axis, the large deshielding in these clusters is thought to be associated with pi(y) and pi z-type orbitals localized on the phosphorus atom of the PPh moiety.