| http://www.w3.org/ns/prov#value | - X3 and X4 independently are a substituted cyclopentadienyl, indenyl, or fluorenyl group, any substituents on X3 and X4 other than a bridging group independently are a hydrogen atom, a methyl group, an ethyl group, a propyl group, a n-butyl group, a t-butyl group, or a hexyl group;
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