New triplet silylenes M–Si–M′–X along with some unusual cyclic forms (M = Li, Na, and K; M′ = Be, Mg, and Ca; X = F, Cl, and Br)

Abstract

Comparison of 54 M-Si--M′′-X species is carried out using quantum mechanical ab initio and DFT computations at B3LYP/6-311++++G**, QCISD(T)/6-311++++G**, and CCSD(T)/6-311++++G** levels of theory (M = Li, Na, K; M′′ = Be, Mg, Ca, and X = F, Cl, Br). All triplet species with M = K appear more linear than their corresponding ones with Li and Na. The electronegativity reactivity descriptor for each halogen (X = F, Cl, Br) is used as a tool to evaluate the interrelated properties of these silylenes. Stability, assumed as singlet-triplet energy difference (ΔES−T)ΔES−T) for each series depends on the substituent's electropositivity, analyzed by applying appropriate isodesmic reactions. Stability of triplet M--Si-M′′-X silylenes increases as functions of electropositivity of αα-substituents and of ββ-substituents. The purpose of the present work was therefore to assess the influence of different di-alkaline metals with different ββ-substituents on the singlet--triplet energy gaps.