The electronic structure and stability of three gas phase heteroatomic clusters, Ga2Bi3-, In2Bi3-, and In2Sb3-, which are isoelectronic with the Zintl ion Sn-5(2-), are examined using photoelectron spectroscopy and first-principles theoretical investigations. While all the isoelectronic clusters are stable with high adiabatic detachment energies, the HOMO-LUMO gap, absolute stability and the relative stability of the isomers depend on the atomic size and point of substitution. Theoretical analysis reveals the variations are attributable to atomic size differences, which affect covalent bonding within the cluster. The studies offer a strategy for controlling properties of clusters that might be incorporated into cluster-assembled materials. (C) 2011 Elsevier B.V. All rights reserved.