Metal-formaldehyde complex

A metal-formaldehyde complex is a coordination complex in which a formaldehyde ligand has two bonds to the metal atom(s) (η2-CH2O). This type of ligand has been reported in both monometallic and bimetallic complexes.

History
Metal-formaldehyde complexes have been reported for tungsten (W), osmium (Os), vanadium (V), rhenium (Re), zirconium (Zr),  ruthenium (Ru), and niobium (Nb).

In 1984, Green and coworkers reported the yellow crystalline solid W(PMe3)4(η2-CH2O)H2. It was the result of the addition of methanol to W(PMe3)4(η2-CH2PMe2)H.

W(PMe3)4(η2-CH2O)H2 can be hydrogenated to give W(PMe3)4(MeO)H3, and then further hydrogenated to reform methanol and generate W(PMe3)4H4. In 1986, Green and Parkin demonstrated further reactivities of W(PMe3)4(η2-CH2O)H2. Upon addition of CO or CO2, W(PMe3)4(η2-CH2O)H2 produces fac-W(PMe3)3(CO)3 and W(PMe3)4(κ2-O2CO)H2, respectively, much like its precursor.

W(PMe3)4(η2-CH2O)H2 also reacts with buta-1,3-diene to give W(PMe3)3(η2-CH2O)(η-C4H6).

W(PMe3)4(η2-CH2O)H2 can also be used as a route to further oxometallacycles by the addition of ethylene and rapid cooling to –80°C. The resultant green-colored crystals are composed of W(OCH2CH2CH2)(PMe3)2(η2-C2H4)2, with either both ethylene ligands on the equatorial plane or the ethylene ligand cis- to the ligating oxygen in the axial direction. Further reaction with ethylene produces trans-W(PMe3)4(η2-C2H4)2 and W(PMe3)4(CO)H2.