{HEAD ~ Y} is {TAIL ~ 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 Z together form a fused ring system 
{ HEAD ~ Y } is { TAIL ~ 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 Z together form a fused ring system
HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH