Carbenes as pure donor ligands: Theoretical study of beryllium-carbene complexes

Quantum chemical ab initio calculations at the MP2/6-31G(d) level of theory are reported for the beryllium-carbene complexes Be(CX₂)_n²⁺ (X = H, F; n = 1-4), ClBe(CX₂)_n⁺ (X = H, F; n = 1-3), and Cl₂Be(CX₂)_n (X = H, F; n = 1, 2). The complex ClBe(C(NH₂)₂)₃⁺ has also been calculated. Where feasible, the bond energies of some molecules are reported at MP4/ 6-311G(d)//MP2/6-31G(d). Analysis of the bonding situation with the help of the natural bond orbital method shows that the carbene ligands are pure donors in the complexes. The dications Be(CX₂)_n²⁺ (X = H, F; n = 1-4) have strong Be²⁺-C donor-acceptor bonds. The bond strengths decrease clearly when the number of ligands increases from n = 1 to 4. The CH₂ complexes have stronger Be-C bonds than the CF₂ complexes. Yet, the CH₂ complexes are chemically less stable than the CF₂ complexes for kinetic reasons. The carbon p(π) orbital of methylene stays nearly empty in the complexes, which makes them prone to nucleophilic attack. All theoretical evidence indicates that the dominant factor which determines the chemical stability of carbenes and carbene complexes is the population of the carbon p(π) orbital. The chemical instability of the methylene complexes becomes obvious by the geometry optimizations of ClBe(CH₂)₂⁺, ClBe(CH₂)₃⁺, Cl₂Be(CH₂), and Cl₂Be(CH₂)₂, which lead to rearranged structures as energy minimum forms. The C-H bonds and particularly the C-F bonds of the ligands are shorter than in free CH₂ and CF₂. The carbon atom of CF₂ becomes electronically stabilized in the complexes via p(π) donation from fluorine. This finding suggests that carbene ligands, which are unstable as free molecules, may become sufficiently stabilized to be isolated even in complexes without metal → carbene back-donation.