An innovative study on the electrochemical behaviour of mixed d metal-iron containing WellsDawson sandwich-type complexes [(FeOH2)(2)M-2(X2W15O56)(2)](14-) and [(MOH2)(2)Fe-2(X2W15O56)(2)](14-) (with M = Cr-III, Mn-II, Mn-III, Co-II, Ni-III, Cu-II or Zn-II and X= P-v or As-v) was carried out. These complexes have a four-centre equatorial metal cluster constituted of two Fe atoms and two atoms of another metal. The Fe-III centres are either in an external position, [(FeOH2)(2)M-2], or in an internal position, [M-2(OH2)(2)Fe-2]. Experimental methods (cyclic voltammetry and controlled potential coulometry) and theoretical calculations (density functional theory) allowed us to determine and analyse the redox potential values associated with the reduction of the Fe-III centres in these species. The influence of the position of the Fe-III centres, the nature of the metal centre M and the electron density distribution in the tetranuclear cluster (either [(FeOH2)(2)M-2] or [(MOH2)(2)Fe-2]) have been studied and rationalised in order to account for the observed behaviours. The data suggest that the most stable isomers are those where Fe-III centres are internally-located, [(MOH2)(2)Fe-2]. Consequently, their reduction is more difficult than those having externally-located Fein isomers, [(FeOH2)(2)M-2]. Some experimental results revealed a few exceptions to this rule which have not been rationalised yet. (C) 2014 Elsevier Ltd. All rights reserved.