AbstractThe clusters InxO (x = 2, 3) and In4O0/—1 arestudied using density functional theory (B3PW91). The global minima contain linear di-coordinated, T-shape tri-coordinated, and planar tetra-coordinated oxygen atoms for In2O, In3O, and In4O, respectively. 18-valence electrons contribute to square structure of In4O. Excitingly, 19-valence electron in [In4O]– contains tetra-coordinated oxygen. Molecular orbitals analysis reveals that delocalized π-bonding and σ-bonding orbitals stabilize the planar tetra-coordinated bonding of oxygen. In addition, the stability becomes worse with a decrease in coordination number.