Extended Huckel molecular orbital calculations, transition metal

Qualitative ideas of catalytic H2 activation on metals were devised in the late 1950s in connection with the formation of transition states or intermediates prior to OA of H2 to hydride complexes (see Chapter 2). Surprisingly, there was no molecular orbital analysis of this intriguing theoretical problem until Dedieu carried one out in 1979.17 Both extended Huckel and ab initio Hartree-Fock calculations were carried out on H2 addition to square-planar d8 RhCl(PH3)3, a model for the well-known Wilkinson catalyst in which the phosphine is PPh3. In this 16e complex, the H2 approaches the filled dzi metal orbital. Calculations indicate that at the beginning of the reaction, end-on (i/1) approach of H-H is preferred over side-on (tf2) approach (Eq. 4.1). 

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