prov:value
| - (c) Non-macro-cyclic ligands of the formula: ##STR6## in which R1, R2, R3 and R4 can each be selected from H, optionally substituted alkyl and aryl groups, and such substituents in which each R1 --N???C--R2 and R3 --C???N--R4 form a five- or six-membered, optionally substituted, nitrogen-containing heterocyclic ring system; and B is a bridging group selected from O, S, CR5 R6, NR7 and C???O, where
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