Journai ofMolecular Structure zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH (Theochem), 121(1985) 121-131 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands zyxwvutsrqponmlkjihgfedc COMPARISON OF THE ACTIVATION OF SMALL COVALENT MOLECULES BY TRANSITION DIMETALS AND MONOMETALS: A THEORETICAL SEMI-EMPIRICAL STUDY P. CHAQUIN*, A. SEWN and H. T. YU Labomtoire de Chimie Organique Th&orique. ** Uniuersitk P. et M. Curie, 4 Place Juseieu, 75230 Paris Cedex 05 Tour 44, ler. (France) (Received 8 June 1984) ABSTRACT The concerted addition of H,, as a model of alkane molecules, to Ni,, as a model of transition dimetal molecules has been studied by Extended Hiickel calculations. Various approaches have been tested and the most likely primary product appears to be M-MH,. Perturbations and MO correlation diagrams are analyzed, emphasizing the striking differ ence found experimentally between mono- and dimetals. INTRODUCTION The recent and fast development of low-temperature matrix chemistry has focused attention on metal atoms or ions as well as reactivity of small metal clusters. Isolated transition metal atoms have been found to be generally unreactive towards activation of small covalent molecules such as dihydrogen or methane [l] . Upon photonic activation, oxidative addition reactions (1) or substitutions (2) are observed M+RH+MRH (1) M+RH+MH+RorMR+H (2) In a previous paper, we have shown that these results can easily be interpreted on the grounds of correlation diagrams supported by Extended Hiickel Theory (EHT) calculations [2]. In contrast, experiments have shown that some dimetal molecules such as Fe? or Ni? are thermally reactive [3] while others such as Vz remain inert under the same conditions [4]. This striking difference from monometal behavior deserves attention since potentially it leads to a bridge between the chemistry of metal atoms and clusters and, in a remote perspective, with chemistry of metal surfaces. Our purpose in this paper is to analyse, in MO correlation terms, the simplest prototype reaction *Author to whom correspondence should be addressed, **The laboratory is part of the UA 506 of the CNRS. 0166-1230/85/$03.30 0 1985 Elsevier Science Publishers B.V.