Various saturated five-membered N,X-heterocyclic carbenes (X = N, O, S, P, Si, C, and B) have been studied by ab initio and density functional theory (DFT) methods. The substitutions alter the properties of the reference carbene from the viewpoint of electronic structure, stability, nucleophilicity, and basicity. Our study shows that the oxygen containing carbene (X = O) induces the highest HOMO–LUMO energy gap (ΔEHOMO–LUMO), while carbene with X = N has the widest singlet–triplet energy difference (ΔEs–t). The nucleophilicity of the carbene derivatives increased upon replacement of C, Si, and B, with the effect of the boron substituent being more pronounced. In addition, the basicity of the structure increased for the carbene derivatives with X = C and B with the latter substitution imposing a remarkably higher effect. Moreover, the substitution of boron at the α-position of the carbene increased the nucleophilicity and basicity, while inducing a reduction in the values of ΔEs–t and ΔEHOMO–LUMO.

Stable configurations of XC 3 H 3 Si five membered rings, 1 X and 2 X (X = CH, N, P, and As) in the singlet and triplet states are found at B3LYP/6 311++G** level of theory. All the singlet states of 1 X and 2 X have lower potential energy than the triplet state. The ΔG s-t differences between the singlet and triplet states of 1 X and 2 X changes at the B3LYP/6 311++G** level in the order (in kcal/mol): 1 N (-17.56) > 1 CH (-15.26) > 1 P (-4.96) > 1 As (-3.45) and 2 CH (-15.26) > 2 N (-9.21) > 2 P (-7.39) > 2 As (-6.15), respectively.