Y is {HEAD ~ an anionic or nonanionic ligand group} bonded to Z and M comprising nitrogen, phosphorus, {TAIL ~ oxygen} or sulfur and having up to 20 non hydrogen atoms, optionally Y and Z together form a fused ring system 
Y is { HEAD ~ an anionic or nonanionic ligand group } bonded to Z and M comprising nitrogen , phosphorus , { TAIL ~ oxygen } or sulfur and having up to 20 non hydrogen atoms , optionally Y and Z together form a fused ring system
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH