The synthesis and characterisation of two diiron hexacarbonyl complexes [Fe 2 (SXS)(CO) 6 ], 1 (SXS=(( − SCH 2 ) 2 C(CH 3 )CH 2 OCOFc, Fc=ferrocenyl group) and 2 (SXS=( − SCH 2 CH 2 NHCOFc) 2 ), were described. By using intramolecularly integrated ferrocenyl group(s) in the complexes as an internal standard, the nature of two stepwise one-electron processes of the complexes coupled with a chemical reaction was clearly demonstrated. Examining how the reduction transformed into sole one-electron process with both increasing scanning rate under Ar/CO atmosphere and lowering temperature indicated conclusively that the reduction of both complexes couples to a chemical reaction which involves CO-loss.