1,3-Diphosphacyclobutane-2,4-diyl-2-ylidenide: A Unique Carbene and Its Trimethylalane Complex

A unique bonding situation is displayed by the lithium 1,3-diphosphacyclobutane-2,4-diyl-2-ylidenide 2 small middle dot[Li(thf)(n)](+) (Ar=2,4,6-tBu(3)C(6)H(2)) obtained by deprotonation of 1. According to ab initio calculations, the anion 2 can viewed as a cyclic bis(phosphanyl)carbene. Reaction with trimethylaluminum gives the complex 3 small middle dot[Li(thf)(4)](+), whose crystal structure is presented.