Comparison of the oxidation of dinuclear cyclopentadienyl iron diphosphine complexes with the bridging ligands -CN and -C C(CH2)(2)CN

The alkynyliron complex [Fe(C=C-CH2CH2-CN)(dppe)(C5H5)] (2) (dppe = Ph(2)PCH(2)CH(2)PPh(2)), prepared from [Fe(C=C-CH2CH2-CN)(CO)(2)(C5H5)] (1) and dppe under UV irradiation, reacted with HBF(4)Et(2)O in tetrahydrofuran and with NH4[PF6] in CH2Cl2 to give the cationic vinylidene derivative [Fe(=C=CH-CH2CH2-CN)(dppe)(C5H5)][A](A = BF4 (3a) or PF6 (3b)), which can be reconverted to 2 with K2CO3 in CH2Cl2. The compound [(C5H5)(dppe)Fe-NC-CH2CH2-C=CH][PF6] (4), which is a tautomeric form of 3 was prepared by reaction of [Fe(I)(dppe)(C5H5)] with the alkyne HC=C-CH2CH2CN in the presence of NH4[PF6] in CH2Cl2. The dinuclear compound [(C5H5)(dppe)Fe-C=C-CH2CH2-CN-Fe(dppe)(C5H5)][PF6] (5), which is unstable, can be formed by reaction of 2 with [Fe(NCMe)(dppe)(C5H5)] PF(6)in CH2Cl2. Its electrochemical oxidation shows that there is no electronic interaction between the two metal centres. Extended Huckel molecular orbital calculations have been carried out on the model complexes [(C5H5)(PH3)(2)Fe-C=C-CH2CH2-CN-Fe(PH3)(2)(C5H5)](+) (7) and [(C5H5)(PH3)(2)Fe-CN-Fe(PH3)(2)(C5H5)](+) (8).