Biferrocenium cation

The piezochromism of [Fe(CN)5(4CNpy)] and [Fe(CN)5(pz)] (and of the biferrocenium cation) was included in a wide-ranging solvatochromism/piezochromism correlation for metal-to-ligand (and ligand-to-metal) charge-transfer bands of a variety of inorganic and organometallic complexes. 

Dong, T.Y., Chang, L.S., Lee, G.H. and Peng, S.M., Pronounced effects of zero-point energy difference on intramolecular electron transfer in asymmetric mixed-valence biferrocenium cations Structural, EPR, and Fe Mossbauer characteristics, Organometallics... 

For example, with the biferrocenium cation below, both infrared bands of the Fe and Fe complexes are observed it belongs to the class 2. On the other hand, with two arene ligands (below, right), the monocation shows infrared absorptions intermediate between those of Fe and Fe complexes it belongs to the class 3. 

The differences between biferrocenium and l,r-biferrocene[Fe(n)-Fe(in)] cations have been previously discussed [57]. It is sufficient to note here that the crystal structure of l,r-biferrocene[Fe(ii)-Fe(in)] picrate [61a] reveals a shorter Fe-Fe distance than that in the neutral compound [61b] (3.64 vs. 3.98 A) indicating increased interaction between the two metal centers in the cation. Possibly because a high yield synthesis of 1,1 -biferrocenylene is not available, and because the derivative chemistry of this compound is essentially undeveloped, no other relevant studies directed toward the preparation of molecular materials based on it have appeared in the literature. 

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