An Isolable Diphosphene Radical Cation Stabilized by Three-Center Three-Electron π-Bonding with Chromium: End-On versus Side-On Coordination.

Two salts (2 and 4) containing the radical cations of complexed diphosphenes have been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, IR spectroscopy, and single-crystal X-ray diffraction. The P-P bond is coordinated to the Cr center either in an end-on (in 2) or a side-on (in 4) fashion. The spin density of the radical is delocalized over the Cr atom and the two P atoms in 2 whereas the unpaired electron is mainly localized on the Cr atom in 4. This work provides the first example of a complexed diphosphene radical (2) featuring novel three-center three-electron (3c-3e) π-bonding in the Cr-P-P unit, and the first example of a 17 e Cr radical with a side-on π-bonded ligand (4).