ACYCLIC CP(CO)2FE DISUBSTITUTED ETA-1-VINYLIDENE-ETA-2-ACETYLENE ISOMERIZATIONS

Acyclic, disubstituted vinylidene complexes, [Cp(CO)2Fe=C=CR1R2]+TfO-, 3a-c (a, R1, R2 = Me; b, R1 = Ph, R 2 = Me; c, R1, R2 = Ph), isomeriz e to nonterminal -acetylene complexes, [CP(CO)2Fe(eta2-R1-C=C-R2)]+TfO -, 5a-c, in CD2Cl2 above -50-degrees-C. Carbon-13 labeling and VT 13C NMR are used to track the formation of 3 and its conversion to 5. The isomerizations are intramolecular and apparently irreversible. That of 3b to 5b involves only phenyl migration. It is suggested that Cp(CO)2 Fe eta2-acetylenes are favored over CP(CO)2Fe eta1-vinylidenes under t hese conditions by the strongly electron withdrawing COs which, relati ve to less pi-acidic ligands, destabilize the vinylidene and stabilize the pi-acetylene.