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