Here we present evidence that the gap between the highest occupied and lowest unoccupied molecular orbitals ( HOMO-LUMO gap) can be tuned (1.12-1.89 eV) by changing the Ga composition of Bi3Gay anionic and neutral clusters, some of which display special stability. Collaboratively, mass spectrometry, photoelectron spectroscopy and computational results show that Bi3Ga2- is a very stable cluster with a large calculated HOMO-LUMO gap of 1.89 eV, and can be viewed as a gas phase Zintl analogue of Sn-5(2-), already synthesized in the solution phase. The stability of Bi3Ga2- is further attributed to the fact that it has 12 valence electrons and possesses a closo structure in agreement with Wade's rules. (c) 2008 Elsevier B. V. All rights reserved.