| http://www.w3.org/ns/prov#value | - one substituent on (X1) and (X2) is a bridging group having the formula ER1R2, wherein E is a carbon atom, a silicon atom, a germanium atom, or a tin atom, and E is bonded to both (X1) and (X2), and wherein R1 and R2 are independently an alkyl group or an aryl group, either of which having up to 12 carbon atoms, or hydrogen, wherein at least one of R1 and R2 is an aryl group;
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