| Summary: | <p>In this paper, we report the synthesis and structural characterisation of two hetero-metallic clusters, [(CO)<sub>3</sub>CrSn<sub>5</sub>Cr(CO)<sub>3</sub>]<sup>4−</sup> and [(CO)<sub>3</sub>MoSn<sub>5</sub>Mo(CO)<sub>3</sub>]<sup>4−</sup>, both of which have a pentagonal bipyramidal core. The structures are similar to that of previously reported [(CO)<sub>3</sub>MoPb<sub>5</sub>Mo(CO)<sub>3</sub>]<sup>4−</sup> and our analysis of the bonding suggests that they are best formulated as containing Sn<sub>5</sub><sup>4−</sup> rings bridging two zerovalent M(CO)<sub>3</sub> fragments. The electronic structure is compared to two isolobal M2E<sub>5</sub> clusters, [CpCrP<sub>5</sub>CrCp]<sup>−</sup> and Tl<sub>7</sub><sup>7−</sup>, both of which show clear evidence for <em>trans</em>-annular bonds between the apical atoms that is not immediately obvious in the title clusters. Our analysis shows that the balance between E-E and M-M bonding is a delicate one, and shifts in the relative energies of the orbitals on the E<sub>5</sub> and M<sub>2</sub> fragments generate a continuum of bonding situations linked by the degree of localisation of the cluster LUMO.</p>
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