Bipyridine or phenanthroline ligands presenting functional groups that prevent non-specific binding (in particular, negatively charged functional groups that are unaffected by standard conditions for conjugating biological reagents through amide bonds) are described as are luminescent metal complexes comprising these ligands. The use of luminescent ruthenium and osmium complexes comprising these ligands in electrochemiluminescence assays shows that the use of these labels can significantly reduce the amount of non-specific binding observed relative to assays carried out using reagents labeled with analogous labels that don't present functional groups that decrease non-specific binding.