Theoretical description of triplet silylenes evolved from H ₂Si=Si

Computations describe the dependence of the H₂M=Si triplet electronic structure on the α-substituent. Whereas silylidenes H ₂C=Si and H₂Si=Si benefit from a π¹p ¹ triplet state, the electronegative nitrogen of HN=Si prefers an n¹p¹ triplet. CCSD(T) and B3LYP calculations predict R₂Si=Si triplet silylenes are stabilized by π-donor/σ- acceptor R substituents which compensate for electron deficiency in the singly occupied π orbital of the π(1)p(1) triplet state. (NH₂) ₂Si=Si, (OH)₂Si=Si, F₂Si=Si, (NH ₂)HSi=Si, and (OH)HSi=Si all are triplet ground states. In particular, (NH₂)₂Si=Si and (OH)₂Si=Si have singlet-triplet energy gaps (ΔE_S-T = E_T - E _S) of -10.2 and -10.3 kcal/mol, respectively. More practical results are achieved via cyclization of (NH₂)₂Si=Si, which eliminates the probability of rearrangement. Unsaturation of the resulting cyclic structure to give (NHCHCHNH)Si=Si leads to a more favorable triplet silylene with a ΔE_S-T value of -19.6 kcal/mol. Similar to the common approach of bulky substitution in the synthesis of singlet Arduengo-type N-heterocyclic silylenes, triplet (NRCH₂CH₂NR)Si=Si and (NRCHCHNR)Si=Si could be experimentally achievable.