A radical species characterized by a large g-anisotropy and a clearly resolved hyperfine structure with ^95/97Mo and ³¹P nuclei is formed, at 77 K, by radiolysis of a single crystal of Mo(CO)₅PPh₃. The corresponding EPR signals disappear irreversibly with increasing temperature and the angular dependence of the various coupling constants imply a spin delocalization of not, vert, ∼60% and not, vert, ∼4% on the molybdenum and the phosphorus atoms, respectively and are, a priori, consistent with the trapping of a one-electron deficient centre. The ability of DFT to predict the EPR tensors for a 17-electron Mo^(I) species is verified by calculating the g-tensor and the various ¹⁴N and ¹³C coupling tensors previously reported by Hayes for [Mo(CN)₅NO]^3-. Calculations at the B3LYP/ZORA/SOMF level of theory show that, in contrast to Mo(CO)₅PH₃, one-electron oxidation of Mo(CO)₅PPh₃ causes an appreciable change in the geometry of the complex. The g-tensor and the ^95/97Mo and ³¹P isotropic and anisotropic coupling constants calculated for [Mo(CO)₅PPh₃]^+· confirm the trapping of this species in the irradiated crystal of Mo(CO)₅PPh₃; they also show that the conformational modifications induced by the electron release are probably hindered by the nearby complexes.

[M(CO)₄PPh₃]^{•−} (M = Mo, W) were trapped at 77 K in X-irradiated single crystals of M(CO)₅PPh₃ and studied by EPR. Structures of [M(CO)₄PPh₃]^{•−} (M = Cr, Mo, W) were optimized by DFT; predicted g and ³¹P-hyperfine tensors agree with experiments for M = Mo, W. The anions adopt a slightly distorted pyramidal structure with PPh₃ in basal position and the spin mostly delocalized in a metal-d_z² orbital and carbon-p_z orbitals of carbonyls. The EPR tensors are slightly modified by annealing, they suggest that new constraints in the matrix distort the structure of [M(CO)₄PPh₃]^{•−} (M = Cr, Mo, W).

Paramagnetic complexes M(CO)₅P(C₆H₅)₂, with M = Cr, Mo, W, have been trapped in irradiated crystals of M(CO)₅P(C₆H₅)₃ (M = Cr, Mo, W) and M(CO)₅PH(C₆H₅)₂ (M = Cr, W) and studied by EPR. The radiolytic scission of a P−C or a P−H bond, responsible for the formation of M(CO)₅P(C₆H₅)₂, is consistent with both the number of EPR sites and the crystal structures. The g and ³¹P hyperfine tensors measured for M(CO)₅P(C₆H₅)₂ present some of the characteristics expected for the diphenylphosphinyl radical. However, compared to Ph₂P^•, the ³¹P isotropic coupling is larger, the dipolar coupling is smaller, and for Mo and W compounds, the g-anisotropy is more pronounced. These properties are well predicted by DFT calculations. In the optimized structures of M(CO)₅P(C₆H₅)₂ (M = Cr, Mo, W), the unpaired electron is mainly confined in a phosphorus p-orbital, which conjugates with the metal d_xz orbital. The trapped species can be described as a transition metal-coordinated phosphinyl radical.