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