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(η5-C5H4PMePh2)(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(η5-C5H5)(CO)3]-. A value of +0.44 V was determined for the electronic effect of the R = [PMePh2]+ group compared to R = H in a C5H4R 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 [FeCp2][B(C6F5)4] were carried out to give the crystalline, dimeric products [M(η5-C5H4PMePh2)(CO)3]2[B(C6F5)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 (1H, 13C, 19F, 31P) 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 η5-C5H5 and η5-C5Me5 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 1H NMR spectrum of this complex in THF at 298 K exhibits characteristic C5H4 resonances at δ 26.08 and 13.62.