Synthesis, Structure and Bonding in Pentagonal Bipyramidal Cluster Compounds Containing a cyclo-Sn5 Ring, [(CO)3 MSn5 M(CO)3 ]4– (M = Cr, Mo)

Abstract

In this paper, we report the synthesis and structural characterisation of two hetero-metallic clusters, [(CO)3 CrSn5 Cr(CO)3 ]4– and [(CO)3 MoSn5 Mo(CO)3 ]4–, both of which have a pentagonal bipyramidal core. The structures are similar to that of previously reported [(CO)3 MoPb5 Mo(CO)3 ]4– and our analysis of the bonding suggests that they are best formulated as containing Sn4 – 5 rings bridging two zerovalent M(CO)3 fragments. The electronic structure is compared to two isolobal M2 E5 clusters, [CpCrP5 CrCp]– and Tl7 – 7, both of which show clear evidence for trans-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 E5 and M2 fragments generate a continuum of bonding situations linked by the degree of localisation of the cluster LUMO.