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