Synthesis and electrochemical aspects of group 14 iron complexes of the type (η5-C5H5)Fe(L)2ER3, L2 = (CO)2, (Ph2P)2CH2; E = C, Si, Ge, Sn

The dicarbonyl and diphosphine complexes of the type (η5-C5H5)Fe(L)2ER3 (L2 = (CO)2 (a), (Ph2P)2CH2 (b); ER3 = CH3 (1a/b); SiMe3 (2a/b), GeMe3 (3a/b), SnMe3 (4a/b)) were synthesized and studied electrochemically. Cyclic voltammetric studies on the dicarbonyl complexes 1a–4a revealed one electron irreversible oxidation processes whereas the same processes for the chelating phosphine series 1b–4b were reversible. The Eox values found for the series 1a–4a were in the narrow range 1.3–1.5 V and in the order Si > Sn ≈ Ge > C; those for 1b–4b (involving replacement of the excellent retrodative π-accepting CO ligands by the superior σ-donor and poorer π-accepting phosphines) have much lower oxidation potentials in the sequence Sn > Si ≈ Ge > C. This latter oxidation potential pattern relates directly to the solution 31P NMR chemical shift data illustrating that stronger donation lowers the Eox for the complexes; however, simple understanding of the trend must await the results of a current DFT analysis of the systems.