Ab initio calculations suggest that a series of clusters of the [Pt-3(mu-L)(3)(L')(3)(mu(3)-H](+) type (L = CO, SO2, CNH; L' = PH3, CNH) are stable. We have studied these clusters at the HF, MP2, B3LYP, PBE and TPSS theory levels. The magnitude of the interaction energies and distances indicates a substantial covalent character of the Pt-3-H bond, confirmed by orbital diagrams. In addition, the Fukui index of electrophilic attack and electrophilicity index on the unprotonated clusters were used to explore possible sites where chemical reactivity may play a role. (C) 2011 Elsevier B. V. All rights reserved.