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The reasons for large changes in the energetics of C1-C5 and C1-C6 (Bergman) cyclizations of enediynes upon one-electron are studied by DFT and coupled cluster computations. The theoretical prediction of large substituent effects in radical anionic cycloaromatization reactions is reported.

Reduction of a solution of octamethylcyclo-di(m-silylphenylenedisiloxane) in THF on a potassium mirror led to EPR/ENDOR spectra characterized by a large coupling with two protons. The structure of octamethylcyclo-di(m-silylphenylenedisiloxane) is a model for an electron transfer transition state between two aromatic rings.

The successive one-electron reduction of a two-phosphinines-containing macrocycle results in the formation of a phosphorus-phosphorus bond.

The radical anion (tmbp)(super .-), where tmbp = 4,4',5,5'-tetramethyl-bisphosphinine, was derived by the reduction of tmbp on a potassium mirror. The crystal structure, EPR/ENDOR spectroscopy and DFT calculations show that one-electron reductions of biphosphinine provoke a drastic change in its molecular structure, which makes it well-suited for stabilization of metals in a low oxidation state.

Research was conducted to examine the barrier to rotation of the P-H group around the C-*P bond using a new phosphine, the diphenyldibenzobarrelenephosphine. The EPR spectrum of the resulting phosphinyl radical, trapped in a single crystal of diphenyldibenzobarrelenephosphine, was examined at 77 K and at room temperature after homolytic scission of a P-H bond by radiolysis. Results indicate that interaction between the phosphinyl hydrogen and the phenyl ring bound to the ethylenic bond explains the potential energy profile.

Paramagnetic complexes trapped in irradiated crystals of M[(CO).sub.5]P[([C.sub.6][H.sub.5]).sub.3] and M[(CO).sub.5]PH[([C.sub.6][H.sub.5]).sub.2] was studied by EPR. It was observed that in the optimized structures of M[(CO).sub.5]P[([C.sub.6][H.sub.5]).sub.2] the unpaired electron is mainly confined in a phosphorous p-orbital which conjugates with the metal [d.sub.xz] orbital.

The interplay between steric and electronic preferences, the Ar-P=P-Ar-P=P- array and their dependence upon its redox state and acting as a molecular switch, are discussed. The structures of two disphosphaalkenes and a bis(diphosphene) and their reduced products are examined by a combination of crystallographic, ERP spectroscopic and calculation methods.

Cyclic voltammetry of Mes* P=C (NMe2) 2 and Mes*P=C (CH3) NMe2 (2) shows that, in solution in DME, these compounds are reversibly oxidized at 395 and 553 mV, respectively. The electronic behavior of inversely polarized phosphaalkenes 1 and 2 is found to be unexpectedly rich.