P-31 CHEMICAL-SHIFT ANISOTROPIES IN SOLID, OCTAHEDRAL CHROMIUM(0) TRIPHENYLPHOSPHINE DERIVATIVES

The P-31 chemical shift anisotropies have been measured at 293 K for t he triphenylphosphine ligands in solid pentacarbonyl(triphenylphosphin e)chromium(0), Cr(CO)(5)(PPh(3)) (1), and cis- and onyl(triphenylphosp hine)(thiocarbonyl)chromium(0), Cr(CO)(4)(PPh(3))(CS) (2 and 3). The m ajor changes in the shift tensors occur for the delta(11) and delta(22 ) components perpendicular to the Cr-P bond direction. The individual tensor components of the P-31 chemical shifts are clearly more importa nt than are the isotropic values in providing information on the chrom ium-phosphorus bonding. The crystal structure of 3 has been determined by single-crystal X-ray diffraction. The complex crystallizes in the monoclinic P2(l)/n (No. 14) space group with cell constants (at 293 K) a = 9.558(5) Angstrom, b = 15.275(2) Angstrom, c = 15.341(2) Angstrom , beta = 96.66(2)degrees, V = 2225(1) Angstrom(3) and Z = 4; R = 0.069 and R(W) = 0.067. The crystal structure of 1 has been reported previo usly and most of the structural features are quite similar to those fo r 3. For instance, the Cr-P distances are 2.422(4) Angstrom for 1 and 2.424(4) Angstrom for 3. The P-Cr-C(S) and Cr-C-S linkages in 3 are al most linear with the angles being 178.4(4) and 174.1(9)degrees, respec tively. The most significant difference between 1 and 3 is that Cr-C(X ) (X = O, S) distance trans to PPh(3) is appreciably shorter for the t hiocarbonyl, viz., 1.79(1) vs 1.845(4) Angstrom. This shortening would be expected if CS is a much better pi-acceptor ligand than is CO, as is thought to be the case.