Carbenes as Pure Donor Ligands:  Theoretical Study of Beryllium−Carbene Complexes¹

Abstract

Quantum chemical ab initio calculations at the MP2/6-31G(d) level of theory are reported for the beryllium−carbene complexes Be(CX2) n 2+ (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) n 2+ (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.

Synthesis and structural characterization of Palladium(II) complex bearing N-Heterocyclic Carbene

“…The bond distances between carbene C atom bonded to Pd and N atoms in the NHC ring are shorter than the other C-N bond distances. This is possibly indicative of a greater partial double-bond character due to partial electron donation by N to the carbene C atom donor [24][25]. Theoretical study also indicates that the stability of these carbenes is due to electron donation from the nitrogen lone pairs into the formally empty p(p) orbital of the carbene C atom (herein C1 and C2) [26].…”

The Nature of the Metal–Carbene Bond in Normal and Abnormal Pyridylidene, Quinolylidene and Isoquinolylidene Complexes

“…[14][15][16] This tendency is amplified upon coordination to a metal centre. [17,18] Aromaticity within the heterocycle also contributes to the NǞC π-donation, so that upon coordination, unsaturated 2-imidazolylidenes are more stable than saturated 2-imidazolidinylidene counterparts. [19] Moreover, even though the difference in relative bond-dissociation energies between such saturated and unsaturated carbene complexes is rather small, their catalytic properties are quite different.…”

Rh(I) and Pd(II) complexes of methoxy functionalized heterocyclic carbene: Synthesis and characterization

“…These results agree with the values of the literatures [16,17]. The C1-N9 and N2-C9 bond distances are 1.322(6) and 1.351(6) Å, respectively, which are significantly shorter than the other C carbene −N bond distances present in the complexes, for instance the N1-C10 is 1.479(6) Å, and is possibility indicative of greater partial double bond character in these C carbene −N bonds due to partial electron donation by nitrogen to the carbene carbon donor [1,19]. The theoretical studies also indicated that the stability of these carbenes is due to electron donation from the nitrogen lone pairs into the formally vacant p(π) orbital of the carbene carbon [18].…”