The structure of diphosphine radical cations

Abstract

When R 2NNR 2 molecules lose an electron to give (R 2NNR 2) +· radical cations, the whole unit becomes planar, with a(π 1) 2(π 2) 1 configuration. However, because R 3P molecules are far more strongly pyramidal than R 3N molecules, this flattening on electron loss is less, and phosphorous centred radical cations do not achieve planarity. This is clearly so for (R 2PPR 2) + centres, whose liquid and solid state spectra analysed herein in terms of two equivalent 31P hyperfine couplings, show ca. 9% 3s character. This indicates considerable bending at each phosphorous centre. Furthermore, the form of the spectra, with no x — y splitting of the ‘perpendicular’ lines, suggests that each 31P coupling shares a common axis. This means that a trans conformation is required, as expected because this relieves steric strain and favours “π” type orbital overlap.