Molecular Structure and Bond Characterization of the Fischer-Type Chromium−Carbene Complexes (CO)₅CrC(X)R (X = H, OH, OCH₃, NH₂, NHCH₃ and R = H, CH₃, CHCH₂, Ph, C⋮CH)

Abstract

In this work we have examined a series of 25 different Fischer carbene complexes of the type (CO)5CrC(X)R with X = H, OH, OCH3, NH2, and NHCH3 and R = H, CH3, CHCH2, Ph, and C⋮CH, to analyze the influence of each substituent on the molecular structure and bonding of these complexes. Energy and charge decomposition analysis of the interaction between the chromium pentacarbonyl and the carbene fragments reveal that for the metal−carbene bond donation is quantitatively more important than back-donation. However, it is the back-donation rather than donation that correlates with most geometrical and electronic parameters of the complexes studied. The reasons for the larger influence of back-donation on the molecular structure of Fischer carbene complexes are discussed.

Phospha-Fischer Carbenes: Synthesis, Structure, Bonding, and Reactions of Pd(0)− and Pt(0)−Phosphenium Complexes

“…For Fischer carbenes, the percentage of ∆Eorbint attributable to -bonding has been reported to lie between 25 and 50 %. 75,83,84 Therefore, the bonding in 14 is clearly of Fischer type also at the quantitative level and, as anticipated, 79 the M-NHP bond is dominated by -backbonding.…”

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

“…77,78,85 The covalent portion consists of some 20% π-bonding, which compares well with the π-character calculated for Fischer-type amino carbene complexes. 86 Both the σ-and π-contributions to the M-C bond have been calculated to be larger in pyridiniumderived NHC complexes than in the corresponding 2-imidazolylidene systems. 77 Although the relative composition of the M-C bond seems to be remarkably insensitive to the position of the heteroatom, the calculated bond strengths vary considerably.…”

“…72 Examination of the molecular structure of this pyridylidene-type complex shows significant alteration of the bond length within the heterocycle, indicating castabilization of the carbene center by the nitrogen and the exocyclic heteroatom. Similarly, the complexes 100 72 and 116 86 have been synthesized by this route, i.e., by alkylation after heterocycle metallation. In all examples, the metalbound carbon resonance shifts significantly downfield upon methylation.…”

Recent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type

“…Some attention is given to spectral data in support of the carbene ligand character, while specific structural aspects are emphasized. Theoretical calculations in the past have focused mainly on the donor/acceptor nature of the carbene substituents of simple monocarbene complexes [12][13][14][15][16][17] or the steric and electronic effects of the heteroatom on the carbene ligand [18].…”