Ligand-Supported E3Clusters (E=Si-Sn)

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.

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.

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

Si_nC_3-n (n ≤ 3) clusters are interstellar species that are transient in nature at ambient conditions. Herein, the structure, stability, and nature of bonding in cyclic alkyl(amino) carbene (cAAC) protected Si_nC_3-n (n ≤ 3) clusters are studied in silico. The Si₃(cAAC)₃ complex was previously reported to be synthesized in large scale. The present results indicate that because the C-C_cAAC bond is stronger than the Si-C_cAAC bond, C₃(cAAC)₃ and SiC₂(cAAC)₃ complexes have significantly larger stability with respect to ligand dissociation than the Si₃(cAAC)₃ complex, while Si₂C(cAAC)₃ has almost the same stability as in the latter complex. Moreover, considering the Si₃(cAAC)₃ complex as a precursor, the hypothetical successive single Si substitution process by a single C atom in Si₃(cAAC)₃ complex is exergonic in nature. The bonding situation is analyzed by employing natural bond orbital (NBO), electron density, and energy decomposition analyses in combination with the natural orbital for chemical valence theory. These studies show that the nature of bonding in C-C_cAAC and Si-C_cAAC bonds differs significantly from each other. The former bonds are best described as an electron-shared double bond, whereas the latter bonds are of donor-acceptor type consisting of two components, Si←C_cAAC σ-donation and Si→C_cAAC π-back-donation. Nevertheless, in the former bonds, covalent character is larger than the ionic one but in the latter bonds the reverse is true. For some Si-C_cAAC bonds, the π-natural orbital cannot be located by the NBO method, presumably because of slightly lower occupancy than the cutoff values, but the electron density analysis confirms that different Si-C_cAAC bonds in a given complex are almost equivalent in terms of electron density distribution. This paper reports an interesting change in bonding pattern when one replaces Si by a C atom in triatomic silicon carbide clusters stabilized by a ligand.

How Far Can One Push the Noble Gases Towards Bonding?: A Personal Account

Noble gases (Ngs) are the least reactive elements in the periodic table towards chemical bond formation when compared with other elements because of their completely filled valence electronic configuration. Very often, extreme conditions like low temperatures, high pressures and very reactive reagents are required for them to form meaningful chemical bonds with other elements. In this personal account, we summarize our works to date on Ng complexes where we attempted to theoretically predict viable Ng complexes having strong bonding to synthesize them under close to ambient conditions. Our works cover three different types of Ng complexes, viz., non-insertion of NgXY type, insertion of XNgY type and Ng encapsulated cage complexes where X and Y can represent any atom or group of atoms. While the first category of Ng complexes can be thermochemically stable at a certain temperature depending on the strength of the Ng-X bond, the latter two categories are kinetically stable, and therefore, their viability and the corresponding conditions depend on the size of the activation barrier associated with the release of Ng atom(s). Our major focus was devoted to understand the bonding situation in these complexes by employing the available state-of-the-art theoretic tools like natural bond orbital, electron density, and energy decomposition analyses in combination with the natural orbital for chemical valence theory. Intriguingly, these three types of complexes represent three different types of bonding scenarios. In NgXY, the strength of the donor-acceptor Ng→XY interaction depends on the polarizing power of binding the X center to draw the rather rigid electron density of Ng towards itself, and sometimes involvement of such orbitals becomes large enough, particularly for heavier Ng elements, to consider them as covalent bonds. On the other hand, in most of the XNgY cases, Ng forms an electron-shared covalent bond with X while interacting electrostatically with Y representing itself as [XNg]+Y-. Nevertheless, in some of the rare cases like NCNgNSi, both the C-Ng and Ng-N bonds can be represented as electron-shared covalent bonds. On the other hand, a cage host is an excellent moiety to examine the limits that can be pushed to attain bonding between two Ng atoms (even for He) at high pressure. The confinement effect by a small cage-like B12N12 can even induce some covalent interaction within two He atoms in the He2@B12N12 complex.