How to Make the σ0π2 Singlet the Ground State of Carbenes

Successful strategies have previously been developed to stabilize the sigma(2)pi(0) singlet states of carbenes, relative to sigma(1)pi(1) triplet states. However, little or no attention has been paid to the stabilization of the sigma(0)pi(2) singlet states. We present two simple strategies to stabilize the sigma(2)pi(0) singlet states of carbenes, relative to both the sigma(2)pi(0) singlet and sigma(1)pi(1) triplet states. These strategies consist of destabilization of the carbene a orbital by two, adjacent, sp(2) nitrogen lone pairs of electrons and stabilization of the carbene 2p-pi orbital by incorporating it into a five-membered ring, containing two double bonds, or into a six-membered ring, containing two double bonds and a sixth atom that has a low-lying empty pi orbital. B3LYP, CASPT2, and CCSD(T) calculations have been performed in order to assess the success of these strategies in creating derivatives of cyclopenta-2,4-dienylidene and cyclohexa-2,5-dienylidene with sigma(0)pi(6) singlet ground states. Differences between the calculated geometries and binding energies of the Xe complexes of the sigma(0)pi(6) singlet ground state of 2,5-diazacyclopentadienylidene (5) and the sigma(2)pi(0) singlet states of CH2 and CF2 are discussed.