Use of organolithium reagents in the synthesis of diiron complexes with bridged and terminal carbene ligands. X-ray structures of [cyclic] (CO)3Fe{.mu.-C(OEt)-.eta.2-C6H4NMe2}Fe(CO)3 and [cyclic] (CO)3Fe{S(CH2)3S}Fe(CO)2{C(OEt)Ph}