Methylene bridge

A methylene bridge can also act as a bidentate ligand joining two metals in a coordination compound, such as titanium and aluminum in Tebbe's reagent.

A methylene bridge is often called a methylene group or simply methylene, as in "methylene chloride" (dichloromethane CH
₂Cl
₂). As a bridge in other compounds, for example in cyclic compounds, it is given the name methano. However, the term methylidene group (not to be confused with the term methylene group, nor the carbene methylidene) properly applies to the CH
₂ group when it is connected to the rest of the molecule by a double bond (=CH₂), giving it chemical properties very distinct from those of a bridging CH
₂ group.

Reactions

Compounds possessing a methylene bridge located between two strong electron withdrawing groups (such as nitro, carbonyl or nitrile groups) are sometimes called active methylene compounds. Treatment of these with strong bases can form enolates or carbanions, which are often used in organic synthesis. Examples include the Knoevenagel condensation and the malonic ester synthesis.