Ligands of low electronegativity in the VSEPR model: The structures of singlet carbenes

Abstract

Equilibrium structures and force constants for skeletal bending from linearity have been calculated, in the MNDO approximation, for twenty five singlet carbenes CX 2. When the substituent X bears neither vacant orbitals nor lone pairs, the force constant becomes steadily more negative as the electronegativity of X increases; when X bears vacant orbitals, the C→ X π bond order and the force constant both increase with the electronegativity of X. When X bears lone pairs, the force constant parallels the HOMO—LUMO gap at linearity. Previous discussions of the structures of singlet carbenes are shown to be inadequate: the reported results support the interpretation in terms of the second-order Jahn-Teller effect of the observed stereochemical inactivity of lone pairs in the presence of ligands of low electronegativity.