Structures of the radical PNSiMe32NPri2, its dimer, cation and chloro derivativeElectronic supplementary information (ESI) available: The final Cartesian coordinates of the computed structures of the diphosphine 2and the corresponding radical 3, a selected list of distances and associated amplitudes of vibrations and the correlation matrix from the electron diffraction least-squares refinements. See http://www.rsc.org/suppdata/dt/b4/b402926g/

Treatment of PClNSiMe32NPri21 with potassium–graphite in thf afforded the colourless, crystalline diphosphine PNSiMe32NPri222 in good yield. Sublimation of 2in vacuoyielded the yellow phosphinyl radical PNSiMe32NPri23, which upon cooling reverted to 2; the latter in C6D6at 298 K was a mixture of racand mesodiastereoisomers. The yellow, crystalline phosphenium salt PNSiMe32NPri2AlCl44 was obtained from 1and ½Al2Cl6in CH2Cl2. By single-crystal X-ray diffraction XRD the structures of the known compound 1and of 2and 4were determined. The structure of the radical 3, formed by the thermal homolytic dissociation of the diphosphine 2, was determined in the gas phase by electron diffraction GED, utilising data from UMP26-31G ab initiocalculations. The model of the molecule in the GED structure analysis was described by a set of internal coordinates and an initial set of Cartesian coordinates from ab initiocalculations, facilitating the structure analysis. The experimental data were found to be consistent with the presence of a single conformer of the radical in the gas phase. The computed standard homolytic dissociation enthalpy of the P–P bond in the corresponding diphosphine 2, corrected for BSSE, 54 kJ mol−1, is substantially reduced compared to the dissociation enthalpy of tetramethyldiphosphine by the reorganisation energies of the fragments that form upon dissociation. The intrinsic energy content of the P–P bond in the diphosphine 2was estimated to be 286 kJ mol−1, in agreement with the results of previous work on a series of crowded diphosphines.