This type of cluster was already known as is the endohedral Ni@Pb (the cage contains one nickel atom).

The electronic spectrum of nickel cytochrome c, for example, is interpreted as showing that the nickel atom is six-coordinated, rather than four coordinated.

The green tetrahydrate has been determined by X-ray crystallography to be octahedral about the central nickel atom, coordinated by four water molecules and two acetate fragments.

While the nickel atom participates in redox reactions, the iron atom is consistently in a Fe(II) coordination state.

The formal oxidation state of the central nickel atom therefore ranges from +II to +IV in the above transformations (see Figure).

This structure is not cyclic; the three nickel atoms are approximately collinear and each pair of them is bridged by two μ oxygen atoms.

Each nickel atom has tetragonally distorted octahedral geometry, caused by the difference in the length of the Ni-O bonds between the bridging and non-bridging oxygens.

Thus, although a nickel atom has, in principle, ten valence electrons (4s 3d), its oxidation state never exceeds four.

The simplest Fe-Ni clusters are of one iron atom and one nickel atom bonded together.

The nickel atoms are organized corresponding to a "normal" spinel arrangement.