Isolation of a monomeric SiO compound 3 as a stable donor-acceptor complex with two different ligands -a σ-donating ligand (pyridine, dimethylaminopyridine, N-heterocyclic carbene) and a donor-acceptor ligand (iminophosphorane)-is presented. The SiO complex 3 is soluble in ordinary organic solvents and is stable at room temperature in solution and in the solid state. Of particular interest, 3 remains reactive and can be used as a stable and soluble unimolecular SiO reagent.

The donor-stabilized sila-β-lactone 1 reacts with CO via a remarkable reversible [2+2]-cycloaddition reaction to form the spiro-cyclic silicon carbonate derivative 2. Furthermore, photolysis of 2 under pressure of CO affords the first persistent SiO -CO cycloadduct 3, presenting a Si O -like structure, which is stabilized by a Lewis donor-acceptor type ligand. As predicted by theoretical calculations, in marked contrast to the thermodynamically stable SiO dimer, the SiO -CO mixed cycloadduct 3 is labile and readily releases CO .