- The reasons underlying the structural stability of the four-connected NbO net and the ordered defect structure of stoichiometric TiO are presented. After showing the failure of an ionic model to account for the structures of these compounds, we show how the electronic structure of these materials explains their unique crystal structures. Analogies to M6X12 clusters prove especially fruitful in this analysis. Both metal-metal bonding and metal-oxide bonding are important in stabilizing these structures. The way in which vacancies enhance metal-metal bonding is discussed. Finally we conclude with a discussion of energetics and compare NbO with other four-connected nets. 1984, American Chemical Society. All rights reserved.