Donor-Acceptor vs Electron-Shared Bonding: Triatomic SinC3-n (n ≤ 3) Clusters Stabilized by Cyclic Alkyl(amino) Carbene

Stabilization of Interstellar CSi2 Species by Donor Base Ligands: L-CSi2-L; L = cAACMe, NHCMe, and PMe3

The donor ligand bonded singlet (L)₂Si₂C containing a bent Si₂C unit in the middle has been studied by theoretical quantum mechanical calculations (NBO, QTAIM, EDA-NOCV analyses) [L = cAAC, NHC, Me₃P]. EDA-NOCV analysis suggests that this Si₂C is possible to stabilize by a pair of donor base ligands. The bond dissociation energy of the Si₂C fragment is endothermic (85-45 kcal/mol) with a sufficiently high intrinsic interaction energy (ΔE_int = -89 to -48 kcal/mol). Fifty percent of the total stabilization energy arises from electrostatic interactions, and nearly 45% is contributed by covalent orbital interaction between Si₂C and (L)₂ fragments in their singlet states. 75-80% of the orbital interaction energy is contributed by two sets of σ-donation L → SiCSi ← L. The π-back-donation is only 15-10%. The dispersion energy is not negligible (3-5%). The interaction energy is highest for 1 (L = cAAC) among three compounds. Additionally, (cAAC)₂Si₂C-Ni(CO)₃ (4) has been studied. The interaction energy between 1 and Ni(CO)₃ is nearly 61 kcal/mol with the major contribution coming from donation of electron cloud from electron rich Si₂C backbone to empty hybrid orbital of Ni(CO)₃ fragment. A sufficiently strong π-back-donation from (OC)₃Ni to Si₂C has also been identified.

Stabilization of group 14 elements E = C, Si, Ge by hetero-bileptic ligands cAAC, MCOn with push-pull mechanism

The stability and bonding of a series of hetero-diatomic molecules with general formula (cAAC)EM(CO)_n , where cAAC = cyclic alkyl(amino) carbene; E = group 14 elements (C, Si, and Ge); M = transition metal (Ni, Fe, and Cr) have been studied by quantum chemical calculations using density functional theory (DFT) and energy decomposition analysis-natural orbital chemical valence (EDA-NOCV). The equilibrium geometries were calculated at the BP86/def2-TZVPP level of theory. The tri-coordinated group 14 complex (1a, 4a, and 7a) in which one of the CO groups is migrated to the central group 14 element from adjacent metal is theoretically found to be more stable when the central atom (E) is carbon. On the other hand, the two-coordinate group 14 element containing metal-complexes (2, 5, 8, 3, 6, and 9) are found to be more stable with their corresponding heavier analogues. The electronic structures of all the molecules have been analyzed by molecular orbital, topological analysis of electron density and natural bond orbital (NBO) analysis at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. The nature of the cAACE and EM bonds has been studied by EDA-NOCV calculations at BP86-D3(BJ)/TZ2P level of theory. The EDA analysis suggests that the bonding of cAACC(CO) can be best represented by electron sharing σ and π interactions, whereas, C(CO)M(CO)_n-1 by dative σ and π interactions. On the other hand, EDA-NOCV calculations suggests both dative σ and π interactions for cAACE and EM(CO)_n bonds of the corresponding Si and Ge analogues having stronger σ- and relatively weaker π-bonds. The topological analysis of electron density supports the closed-shell interaction for the Si and Ge complexes and open-shell interaction for the carbon complexes. The calculated proton affinity and hydride affinity values corroborated well with the present bonding description. This class of complexes might act as efficient future catalysts for different organic transformations due to the presence of electron rich group 14 element and metal carbonyl.

Revisiting the Bonding Scenario of Two Donor Ligand Stabilized C2 Species

Quantum chemical calculations using density functional methods were performed for complexes of type L₂C₂ with L = NHC^Me (1), SNHC^Me (2) (S = saturated), cAAC^Me (3), and diamidocarbene (DAC^Me) (4). The equilibrium structures of 1-4 possess almost linear C₄ cores. A high thermochemical stability of the complexes with respect to dissociation, L₂C₂ → C₂ + 2L, is indicated by the large bond dissociation energy following the order 3 > 4 > 2 > 1. The results show that the use of SNHC^Me and DAC^Me as ligands is preferable over NHC^Me. The bonding analysis using charge and energy decomposition methods reveals that (cAAC^Me)₂C₂ and (DAC^Me)₂C₂ possess genuine cumulene C₄ moieties, which results from the electron-sharing bonding between quintet L₂ and quintet C₂ fragments. In contrast, the bonding in (NHC^Me)₂C₂ and (SNHC^Me)₂C₂ comes from a combination of dative and electron-sharing interactions between doublet L₂⁺ and doublet C₂^- fragments.

Stabilization of Linear C3 by Two Donor Ligands: A Theoretical Study of L-C3 -L (L=PPh3 , NHCMe , cAACMe )*

Quantum chemical studies using density functional theory and ab initio methods have been carried out for the molecules L-C3 -L with L=PPh3 (1), NHCMe (2, NHC=N-heterocyclic carbene), and cAACMe (3, cAAC=cyclic (alkyl)(amino) carbene). The calculations predict that 1 and 2 have equilibrium geometries where the ligands are bonded with rather acute bonding angles at the linear C3 moiety. The phosphine adduct 1 has a synclinal (gauche) conformation whereas 2 exhibits a trans conformation of the ligands. In contrast, the compound 3 possesses a nearly linear arrangement of the carbene ligands at the C3 fragment. The bond dissociation energies of the ligands have the order 1<2<3. The bonding analysis using charge and energy decomposition methods suggests that 3 is best described as a cumulene with electron-sharing double bonds between neutral fragments (cAACMe )2 and C3 in the respective electronic quintet state yielding (cAACMe )=C3 =(cAACMe ). In contrast, 1 and 2 possess electron-sharing and dative bonds between positively charged ligands [(PPh3 )2 ]+ or [(NHCMe )2 ]+ and negatively charged [C3 ]- fragments in the respective doublet state.