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