The iron heteropenta- and heterooligometallic iron clusters cover huge fields, as shown by a survey covering the crystallographic and structural data of over 470 examples. About 5% of those clusters exist as distortion isomers and are summarized. Those are discussed in terms of coordination around the metal atoms, bond length and bond angles. The isomers are crystallized in triclinic, monoclinic, orthorhombic and tetragonal systems, of which triclinic and monoclinic are the most common. The isomers were found in the series of pentameric (x5) hexameric (x9), heptameric (x5) and octameric (x4) clusters. There are 16 different metal parameters with the iron (Fe) atom, non-transition (Li, Ge, Sn, Sb, Tl, Cd, Hg and Te) and transition (Mo, Ru, Rh, Os, Ir, Pt, Co and Au). The shortest metal-metal bond distances are—2.621(1)Å (Fe– Fe); 2.347(2)Å (Fe–Ge); 2.533(4)Å (Fe–Au); 2.334(5)Å (Ge–Co) and 2.434(50Å (Co–Co).

Systematic studies in the field of stereoselectivity of heterometallic clusters over the last five decades are of immense interest to the scientists. Stereoselectivity in complex compounds (cluster included) is very often related to important stereospecifity of biological systems, catalysis and stereochemical effects in technical processes. Isomers can be broadly classified into two main categories—structural and stereoisomers. The former can be divided into ionization, hydrate, coordination number, linkage and polymerization sub-categories; and the latter can be divided into geometric (cis-trans, fac-mer), optical and distortion isomerism.

Interestingly, in the series of iron heterometallic dimeric (Melník and Kohútová, 2010), trimeric (Melník and Kohútová, 2011) and tetrameric (Melník and Sprusansky, 2011) clusters, only distortion isomers have been found. Almost 200 iron heteropentameric (Melník et al., 2010), 160 iron heterohexa- and heptameric (Melník and Holloway, 2011) and 130 examples of iron heteroocta- and oligomeric (Melník et al., 2009) clusters have been surveyed with almost 30 isomeric examples and discussed with those found in iron hetero-dimeric, -trimeric and -tetrameric clusters.

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