Theoret. chim. Acta (Bed.) 27, 339--354 (1972) 9 by Springer-Verlag 1972 A Theoretical Study of the Interaction of Ethylene with Transition Metal Complexes E. J. Baerends, D. E. Ellis*, and P. Ros Scheikundig Laboratorium, Vrije Universiteit, Amsterdam, The Netherlands Received July 3, 1972 In order to be able to describe the ethylene molecule bonded to an active site of a Titanium- or Nickel fluoride crystal, we have used the Hartree-Fock-Slater model, implemented by a Discrete Variational Method, as introduced by Ellis and Painter. The study of the ethylene molecule at a crystal surface then gives a clear, easily interpreted picture of the electronic structure. The n-back donation from metal to olefm is found to be extremely important, both in the Ti- and in the Ni-com- plex. This back donation is caused by a strong interaction of a d orbital of the central ion with a or* as well as the g* molecular orbital of ethylene. As a result of these interactions, the C-C bond of ethylene is weakened considerably. A comparison is made between the Ti-ethylene and the Ni-ethylene systems. Die Elektronenstruktur yon ~tbylen, das an ein aktives Zentrum eines Titan- bzw. Nickel- fluoridkristalls gebunden ist, ist mittels der Hartree-Fock-Slater-Theorie kombiniert mit der sog. Discrete Variational Method theoretisch behandelt worden. Es zeigt sich, dab die ~-Backdonation sowohl im Nickel- als auch im Titankomplex auBerordentlich wichtig ist; sie entsteht durch eine starke Wechselwirkung eines Metall-d-Orbitals sowohl mit einema*- als auch einem n*-Orbital des ~thylens. Dies ftihrt zu einer erheblichen Schw~ichung der Doppelbindung. AuBerdem werden die beiden Systeme (Titan und Nickel) miteinander verglichen. 1. Introduction The metal olefin bond in transition-metal n-complexes is very interesting, both because of its wide occurrence in organometallic chemistry and because of the special character of this type of chemical bond. The bonding in these n- complexes can be explained in terms of a a-bond, which is formed through elec- tron donation from the olefin to the metal, and a re-bond which is formed through back donation of electrons from the metal to the olefin. Although this model, due originally to Dewar [1] and Chattet al. [21 has been generally accepted, there is still quite some uncertainty as to the extent to which each of these bond- ing mechanisms, a-donation and n-back donation, is operative. Recently, this problem has been extensively discussed in the literature [3-6]. Not only can quantum chemical calculations shed some light on the intrica- cies of the metal olefin bond, they should also aim at a better understanding of the extremely important role of transition metals in olefin chemistry. In parti- cular, catalysis of many reactions of olefins by transition metals is very intri- guing. , Alfred P. Sloan Research Fellow, permanent address: Department of Physics, Northwestern University, Illinois, USA. 23*