Iron–nickel clusters

[1] Individually, iron (Fe) and nickel (Ni) generally form metal clusters with π-acceptor ligands.

[1] Corresponding bulk systems of Fe and Ni atoms show a variety of composition-dependent abnormalities and unusual effects.

[6] The average magnetic moment (μav) increases in a Fe–Ni cluster through the replacement of more and more Fe atoms.

Below is a table of the bond length (Re, in Å), binding energy (Eb, in eV), and magnetic moment (M, in μa) of the small clusters Fe2, Ni2, and FeNi from two authors.

Notice how both authors show that Fe2 has the smallest bond length, the lowest binding energy, and the largest magnetic moment of the cluster combinations.

Below is another table of bond length (Re), binding energy (Eb), and magnetic moment (M) of Fe–Ni clusters containing five atoms.

The magnetic properties of metal clusters are strongly influenced by their size and surface ligands.

Magnetic moments approach bulk values as cluster size increases, though this is often difficult to predict computationally.

Furthermore, Ni-CO π backbonding leaves Ni slightly positive, causing more transfer of electrons to 3d-derived orbitals, which are less disperse than those of 4s.

Together, these effects result in a 3d10, diamagnetic character of the ligated Ni atoms, and their magnetic moment decreases to zero.

The non-bridging sulfur ligands are often cystine amino acid residues that attach the active site to the protein backbone.

[12] The majority of hydrogen currently produced comes from natural gas reformation, and hence does not help remove fossil fuel as an energy source.

[11] Catalysts inspired by the Fe–Ni active site of many hydrogen producing enzymes are particularly desirable due to the readily available and inexpensive metals.

To date, only one example of a Fe–Ni model complex that is stable enough to withstand the range of electronic potential required for catalysis has been published.

[11] By preserving these traits of the enzyme active site, it is hoped that the synthetic complexes will operate at the electrochemical potential necessary for catalysis, have a high turnover frequency and be robust.

Figure 1: Closed triangulated polyhedra. (a) Tetrahedron (T d ), (b) Trigonal bipyramid (D 3h ). (c) Octahedron (O h ). (d) Pentagonal bipyramid (D 5d ). (e) Capped octahedron (C s ). (f) Octadecahedron (C 2r )
Figure 2: Small-molecule arrangements
Figure 3: Geometry of Fe–Ni five-atom clusters
Figure 2:Active site of the Fe–Ni active site in (A) the inactive oxidized and (B) the active reduced form. [ 11 ]
Figure 3:Only reported complex that shows catalytic reduction of H to H 2 [ 14 ]