Site preferences in hetero-metallic [Fe 9-x Ni x ] clusters: a combined crystallographic, spectroscopic and theoretical analysis.

The reaction of mixtures of Fe(O2 CMe)2 ·2H2 O and Ni(O2 CMe)2 ·4H2 O of various compositions with di-2-pyridyl ketone (py2 CO, dpk) in MeCN under an inert atmosphere afforded a family of hetero-metallic enneanuclear clusters with general formula [Fe9-x Nix (μ4 -OH)2 (O2 CMe)8 (py2 CO2 )4 ] (2, x = 1.00; 3: x = 6.02; 4, x = 7.46; 5, x = 7.81). Clusters 2-5 are isomorphous to the homo-metallic [Fe9 ] cluster (1) previously reported by some of us, and also isostructural to the known homo-metallic [Ni9 ] cluster. All four clusters contain a central MII ion in an unusual 8-coordinate site and eight peripheral MII ions in distorted octahedral environments. The distribution of FeII and NiII ions over these two distinct coordination sites in 2-5 can be established through a combination of X-ray fluorescence and Mössbauer spectroscopies, which show that FeII preferentially occupies the unique 8-coordinate metal site while NiII accumulates in the octahedral holes. Density functional theory indicates that the distribution of ions across the two sites arises not from any intrinsic preference of the FeII ions for the 8-coordinate sites, but rather because of the large ligand field stabilization energy available to NiII in octahedral coordination.