Diphosphirane and 1,3-diphosphaallyl anion: geometry and bonding

The unsymmetrical diphosphirane 2a, obtained by addition of dichlorocarbene to the corresponding diphosphene ArPPTsi (Ar  2,4,6,-tri-tert-butylphenyl; Tsi  tris(trimethylsilyl)methyl) presents a structure with the shortest PP bond (2.117 Å) ever observed in the diphosphirane series, and a torsion angle (Ar)CPPC(Tsi) close to 180° (177.0°).