The present invention relates to improved covalent coupling of two or more entities such as biomolecules, polymer compositions, organic/inorganic molecules/materials, and the like, including their combinations, through one or more novel reactive groups attached to linker groups of 2-1000 atoms length. The present invention also contemplates the use of bifunctional bridge molecules to link two or more entities, wherein the functional groups of the bridge molecules are the novel reactive groups of the present invention.