The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar–P=C=C=P–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent ³¹P nuclei. Additional couplings with two equivalent ¹³C are observed with Ar–P=¹³C=¹³C=P–Ar. The ³¹P anisotropic coupling constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initioSCI) were performed on the various isomers of the two radical anions: [H–P=C=C=P–H]^{•–} and [H–P=CH–CH=P–H]^{•–}. Although the optimized geometries of these two species are clearly different, neither of them leads to¹³C/³¹P hyperfine tensors in conflict with the experimental results. The absence of any ¹H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar–P=C=C=P–Ar]^{•–} more probable. 

Fluoren-9-ylidenemethylene-(2,4,6-tri-tert-butyl-phenyl)phosphane (2), a new type of phosphaallene with the terminal carbone incorporated in a cyclopentadienyl ring, has been synthesized and its crystal structure has been determined. The ³¹P and ¹³C (central carbon) hyperfine tensors of the reduction compound of this phosphaallene have been measured on the EPR spectra recorded after electrochemical reduction of a solution of 2 in THF. Structures of the model molecules HP=C=Cp (where Cp is a cyclopentadienyl ring), [HP=C=Cp]√^− and [HP---CH=Cp]√ have been optimized by DFT and the hyperfine couplings of the paramagnetic species have been calculated by DFT and SCI methods. The comparison between the experimental and the theoretical results shows that, in solution, the radical anion [2]√^− is readily protonated and that the EPR spectra are due to the phosphaallylic radical.

Cyclic voltammetry shows that monophosphaallene ArPCC(C6H5)2 (where Ar = C6H2tBu3-2,4,6), 1a, undergoes irreversible reduction at 2266 mV in THF. The EPR spectra of the reduction products are obtained in liquid and frozen solutions after specific 13C enrichment of the allenic carbon atoms. The resulting hyperfine tensors are compared with those obtained from ab initio MP2, MCSCF, CI, and DFT calculations for the radical anion (HPCCH2)-• and for the monophosphaallylic radical (HP•âˆ’CHCH2) ↔ (HPCH−•CH2). The most elaborate treatments of the hyperfine structure (CI and DFT) indicate that the species observed by EPR is the monophosphaallylic radical.

Liquid phase EPR spectra of a diphosphaallenic radical anion have been Recorded after electrochemical reduction of a solution of ArPCPAr in THF at 293 K (Ar = 2,4,6-But3C6H2). The hyperfine coupling interactions of two 31P and one 13C nuclei (in the case of Ar13CPAr) are discussed in the light of AM1 calculations carried out on (ArPCPAr)–, of ab initio calculations performed on the model radical anion (HPCPH)– at the MP2 and MCSCF levels of theory and of DFT calculations on (HPCPH)–. The structure of the radical anion is compared with that of the neutral molecule.

The crystal structure of the title compound, [Pd2Br2(C25H34P)2], a new binuclear phospha-alkene compound containing a trivalent P atom shows a centrosymmetric dimeric arrangement. The Pd2Br2 core is planar and adopts an irregular diamond shape. The coordination of the Pd atom is square planar. No stacking interactions were observed in the molecular packing.

Cyclic voltammetry of phosphafulvene and dibenzophosphafulvene shows that in DMF these compounds are reduced at -1.200 and -1.349 V, respectively. The EPR spectra of the corresponding radical anions, formed by electrochemical reduction or by reaction on a potassium mirror, are Recorded between 110 K and room temperature. The g and 31P hyperfine tensors are measured and compared to those previously obtained for a phosphaalkene radical anion. Abinitio investigations on model phosphaalkene and phosphafulvene radical anions show that, in accord with the experimental results, the electronic structure of these two species are quite different: whereas the unpaired electron is delocalized on the whole PC(H)R moiety in the phosphaalkenic anion, it is markedly localized on the phosphorus atom in the phosphafulvene anion. Calculated spin densities and charge distributions for phosphafulvene and azafulvene anions are compared.

The isotropic hyperfine coupling constants of the diphosphaalkene radical cation have been measured by EPR spectroscopy after electrochemical oxidation of ArP]] C]] PAr (and ArP]] 13C]] PAr) in tetrahydrofuran (THF). The two 31P constants as well as the 13C coupling are close to 90 MHz. Taking HPCPH as a model compound, the structure has been assessed, by extensive ab initio calculations including correlation effects at the MP2 and MCSCF levels of theory. It is found that oxidation of the allenic ]P]] C]] P] structure leads to the formation of two rotamers with HPPH dihedral angles of 458 and 1358. These two structures are compatible with the Jahn–Teller distortion of allene. The calculated hyperfine constants support the EPR results.