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New isolable dialkylsilylene and its isolable dimer that equilibrate in solution.

The new isolable dialkylsilylene 3 bearing a bidentate alkyl substituent was synthesized. Recrystallization of silylene 3 gave yellow crystals of 3 and orange-red crystals of tetraalkyldisilene 4, a dimer of 3. In the solid state, 3 exists as a monomer with a closest distance of 6.745 Å between dicoordinate silicon atoms, while disilene 4 has a remarkably long Si═Si double bond distance of 2.252 Å. An equilibrium between 3 and 4 in solution was observed by NMR and UV-vis spectroscopies, and the thermodynamic parameters of the equilibrium were estimated to be ΔH = -36 ± 3 kJ mol(-1) and ΔS = -170 ± 15 J mol(-1) K(-1). Analysis of the percent buried volume, a measure of the steric demand around the divalent silicon, showed that the flexible steric bulkiness of the alkyl substituent of 3 and 4 allows the reversible dimerization of silylene 3 to disilene 4 and the isolation of both species.