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

SinC3-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 SinC3-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-CcAAC bond is stronger than the Si-CcAAC 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-CcAAC and Si-CcAAC 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←CcAAC σ-donation and Si→CcAAC 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-CcAAC 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-CcAAC 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.