Electron paramagnetic resonance study of the [W(CO)4{P(OMe)3}]− radical anion trapped in a single crystal of [N(PPh3)2][W(CO)4H{P(OMe)3}]

Abstract

Gamma-irradiated single crystals of [N(PPh₃)₂][W(CO)₄H{P(OMe)₃}] contain an anisotropic e.p.r. signal which exhibits hyperfine coupling to a single ³¹P nucleus. X-Ray crystallography showed that the crystals belonged to the orthorhombic system (Pbca), with a= 15.562 6(21), b= 19.980(3), c= 27.161(4)Å, and Z= 8; the undamaged anion had an approximately octahedral geometry in which the H atom was cis to the phosphite ligand. Crystallographically aligned specimens were used to determine both the g and the ³¹P hyperfine interaction tensors [g= 2.0708, 2.0740, 1.9914; a(³¹P)= 75.1, 68.4, 45.5 MHz] and their direction cosines in the crystal-axis system. The form of the g tensor and the alignment of g_min. with the H–W–C_ax. direction of the undamaged anion point to considerable unpaired spin density in the W 5d_z² orbital. Alignment of the ³¹P tensor along g_min. and the W–P bond is attributed to direct overlap of the P 3p_z orbital with the metal atomic orbital. Comparison with other ³¹P hyperfine measurements suggests that e.p.r. provides a good indication of the efficiency of back-bonding in such organometallic radicals.