One-Electron Oxidations of the Methyldiphenylphosphonium Cyclopentadienylide Complexes M(η⁵-C₅H₄PMePh₂)(CO)₃ (M = Cr, Mo, W):  Formation and Dimerization of the 17-Electron, Metal-Centered Radicals [M(η⁵-C₅H₄PMePh₂)(CO)₃]⁺

Electrochemical oxidations of the group 6 metal complexes M(η 5 -C 5 H 4 PMePh 2 )(CO) 3 (M = Cr ( 1 ), Mo ( 2 ), W ( 3 )) occur via one-electron processes at potentials that are highly positive of the values previously observed for the analogous 18-electron anions [M(η 5 -C 5 H 5 )(CO) 3 ] - . A value of +0.44 V was determined for the electronic effect of the R = [PMePh 2 ] + group compared to R = H in a C 5 H 4 R ligand, making it one of the most strongly electron-withdrawing substituents known. On the basis of the experimentally determined oxidation potentials and observations that the oxidized species were long-lived, bulk oxidations of 1, 2, and 3 by [FeCp 2 ][B(C 6 F 5 ) 4 ] were carried out to give the crystalline, dimeric products [M(η 5 -C 5 H 4 PMePh 2 )(CO) 3 ] 2 [B(C 6 F 5 ) 4 ] 2 (M = Cr ( 1 2 2+ ), Mo ( 2 2 2+ ), and W ( 3 2 2+ )). These were isolated analytically pure and were characterized by IR (solid state and solution) and NMR ( 1 H, 13 C, 19 F, 31 P) spectroscopy, high-resolution mass spectrometry, and crystallographically. The dimers all contain metal−metal bonds that are comparable in length with or longer than the metal−metal bonds in the isoelectronic, neutral η 5 -C 5 H 5 and η 5 -C 5 Me 5 analogues, and the metal−metal bond in 1 2 2+ is the longest nonbridged Cr−Cr bond known. As a result of the apparent weakness of its Cr−Cr bond, 1 2 2+ dissociates significantly in solution to the paramagnetic radical cation 1 +; the 1 H NMR spectrum of this complex in THF at 298 K exhibits characteristic C 5 H 4 resonances at δ 26.08 and 13.62.