Cyclic voltammetry of Mes*PdC(NMe 2 ) 2 (1) and Mes*PdC(CH 3 )NMe 2 (2) shows that, in solution in DME, these compounds are reversibly oxidized at 395 and 553 mV, respectively. Electrochemical oxidation or reaction of 1 (or 2) with [Cp 2 Fe]PF 6 leads to the formation of the corresponding radical cation, which was characterized by its electron paramagnetic resonance (EPR) spectra. Experimental 31 P and 13 C isotropic and anisotropic coupling constants agree with density functional theory (DFT) calculations showing that the unpaired electron is strongly localized on the phosphorus atom, in accord with the description Mes*P • -(C(NMe 2 ) 2 ) + . Electrochemical reduction of 1 is essentially irreversible and leads to a radical species largely delocalized on the C(NMe 2 ) 2 moiety; this neutral radical results from the protonation of the phosphorus atom and corresponds to Mes*(H)P-• C(NMe 2 ) 2 . No paramagnetic species is obtained by reduction of 2. The presence of the amino groups, responsible for the inverted electron distribution at the P-C double bond (P --C + ), confers on 1 and 2 redox properties that are in very sharp contrast with those observed for phosphaalkenes with a normal π electron distribution (P + -C -): no detection of the radical anion but easy formation of a rather persistent radical cation. For 1, this radical cation could even be isolated as a powder, 1 •+ PF 6 -. As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphoruscarbon bond caused by the presence of the amino groups.

The radical cation of the redox active ligand 3,4-dimethyl-3 0 ,4 0 -bis-(diphenylphosphino)-tetrathiafulvalene (P2) has been chemically and electrochemically generated and studied by EPR spectroscopy. Consistent with DFT calculations, the observed hyperfine structure (septet due to the two methyl groups) indicates a strong delocalization of the unpaired electron on the central S 2 CQCS 2 part of the tetrathiafulvalene (TTF) moiety and zero spin densities on the phosphine groups. In contrast with the ruthenium(0) carbonyl complexes of P2 whose one-electron oxidation directly leads to decomplexation and produces P2 1 , one-electron oxidation of [Fe(P2)(CO) 3 ] gives rise to the metal-centered oxidation species [Fe (I) (P2)(CO) 3 ], characterized by a coupling with two 31 P nuclei and a rather large g-anisotropy. The stability of this complex is however modest and, after some minutes, the species resulting from the scission of a P-Fe bond is detected. Moreover, in presence of free ligand, [Fe (I) (P2)(CO) 3 ] reacts to give the complex [Fe (I) (P2) 2 (CO)] containing two TTF fragments. The two-electron oxidation of [Fe(P2)(CO) 3 ] leads to decomplexation and to the P2 1 spectrum. Besides EPR spectroscopy, cyclic voltammetry as well as FTIR spectroelectrochemistry are used in order to explain the behaviour of [Fe(P2)(CO) 3 ] upon oxidation. This behaviour notably differs from that of the Ru(0) counterpart. This difference is tentatively rationalized on the basis of structural arguments.

We report the synthesis, crystal structure and electrochemical behaviour of a complex in which the Ph group of the phosphaalkene PhC(H)@PMes* (Mes*: 2,4,6-tri-tert-butylphenyl) is coordinated to a chromium tricarbonyl group. The EPR spectra resulting from electrochemical and chemical reductions are described and the experimental g and hyperfine tensors ( 31 P)T, as determined from the EPR data, are compared with those predicted by DFT calculations for the radical anion (Cr(CO) 3 , PhC(H)@PMes) ÅÀ . The structural changes caused by the addition of an electron to the neutral complex are described, together with an estimation of the contribution of Cr(CO) 3 to the stabilization of the radical anion.

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