Donor-substituted phosphanes – surprisingly weak Lewis donors for phosphenium cation stabilisation

Paradoxically, N- and O- donor substituted tri-arylphosphanes are shown to be weaker donors than PPh\(_3\) when binding the soft Lewis acid moiety [PPh\(_2\)]\(^+\). This arises from internal solvation and rehybridisation at phosphorus, precluding chelation and increasing steric demand, in direct contrast to coordination modes observed for metal complexes