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