z represents an integer less than or equal to {HEAD ~ the available coordination bonding sites} of {TAIL ~ the Fe} 
of
z represents an integer less than or equal to { HEAD ~ the available coordination bonding sites } of { TAIL ~ the Fe }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL