Most binary materials formed by early group 13 and 15 elements are semiconductors identical in their structures and properties. We show that in the ultra-small cluster regime these materials are markedly different. Among X3Y3 (X = B, Al, Ga; Y = P, As), lighter clusters are planar, and heavier clusters are compact and three-dimensional. The difference is owed to the interplay between covalency and delocalized non-directional bonding in these systems. If the sp-hybridization in the constituent elements is energetically affordable and leads to strong directional overlap, the bonding is covalent and the cluster is flat. Otherwise, the cluster is three-dimensional. (C) 2013 Elsevier B. V. All rights reserved.