({ }^{57} \mathrm{Fe}) Mössbauer investigations are carried out on a wide series of heterometallic diaquadiformato (\mathrm{Fe}(\mathrm{II}) / M) (II) complexes with (M=\mathrm{Mn}, \mathrm{Co}, \mathrm{Ni}, \mathrm{Cu}), and (\mathrm{Zn}) to provide a local picture of the coordination environment of the ({ }^{57} \mathrm{Fe}^{2+}) ions as a function of (i) the nature of the host cation and (ii) the relative amounts of both metals in the matrix (between 50 and 0.25 at.% Fe). Information is obtained on the quantitative distribution of both metals between the 1wo structurally nonequivalent lattice sites and on the local geometry around the dopant atom in each crystal site. In the mixed (\mathrm{Fe}-\mathrm{Cu}) complexes, (\mathrm{Fe}^{2+}) ions are preferentially incorporated in the tetrahydrated site; in (\mathrm{Cu}-) rich (\mathrm{Fe}{x} \mathrm{Cu}{1-x}\left(\mathrm{HCO}{2}\right){2} \cdot 2 \mathrm{H}_{2} \mathrm{O}), the ({ }^{57} \mathrm{Fe}^{2+}) ions located in the hexaformato-coordinated site are surrounded by an axially compressed octahedron of formate ligands which contrasts with the elongated configuration observed in the pure iron compound and in the other mixed systems. Semiquantitative estimations of the tetragonal field splitting and of the extent of metal-ligand interactions are proposed from the temperature dependence of the quadrupole splitting values. 1993 Academic Press. Inc.