ANISOTROPY IN THE HG-199-P-31 INDIRECT SPIN-SPIN COUPLING TENSOR OF A 1/2-MERCURY-PHOSPHINE COMPLEX - A PHOSPHORUS SINGLE-CRYSTAL NMR-STUDY

A single crystal of the 1:2 mercury-phosphine complex, Hg(PPh(3))(2)(NO3)(2), has been investigated by phosphorus-31 NMR spectroscopy. In the solid state, this species possesses two magnetically-distinct phosphorus nuclei which are spin-spin coupled to one another, resulting in P-31 NMR spectra which exhibit A(2), AB, or AX character depending upon the orientation of the single crystal in the external magnetic field. The three principal components of the phosphorus chemical shift (CS) tensors as well as their orientations in the molecular reference frame have been obtained from analyses of the single-crystal P-31 NMR spectra. The most shielded component of the CS tensor (delta(33) = 15 ppm) lies approximately 14 degrees off the Hg-P bond axis while the least shielded component (delta(11) = 72 ppm) is oriented within the plane containing the smallest Hg-P-C bond angle. These findings represent the first characterization of a phosphorus CS tensor for a 1:2 mercury-phosphine complex. The absolute sign of the two-bond indirect spin-spin coupling, (2)J((31)p,(31)p)(iso), has been determined to be positive. Analyses of the orientation dependence of the mercury-199 (I = 1/2, natural abundance = 16.84%), phosphorus-31 spin-spin interactions reveals that the Hg-199-(31)p J tensor is anisotropic with J(parallel to) = 8.2 kHz and J(perpendicular to) = 4.2 kHz. This result indicates that indirect spin-spin coupling mechanisms other than Fermi contact are operative. These findings complement an earlier characterization of the Hg-199-(31)p J tensor in the 1:1 mercury-phosphine complex, HgPCy(3)(NO3)(2) (Cy = cyclohexyl). For both the 1:1 and 1:2 species, the anisotropy in J is appreciable relative to the isotropic coupling.