DFT-studies of cis - and trans -[Rh(CO) 2 X 2 ]  (X /PH 3 , PF 3 , PCl 3 , PBr 3 , PI 3 or P(CH 3 ) 3 ) and oxidative addition of CH 3 I to them Tapani Kinnunen, Kari Laasonen * Department of Chemistry, University of Oulu, P.O. Box 3000, Oulu FIN-90014, Finland Received 15 November 2002; received in revised form 15 November 2002; accepted 17 November 2002 Abstract We have performed B3LYB density functional calculations for oxidative addition of CH 3 I to [Rh(CO) 2 X 2 ]  (X /PH 3 , PF 3 , PCl 3 , PBr 3 , PI 3 or P(CH 3 ) 3 ). This species is similar to the catalyst used in industrially important Monsanto process. Our goal is to see if the catalytic process could be enhanced by modifying the ligands. We have taken a set of phophine ligands and calculated the oxidative addition of methyl iodide to both the cis - and trans -forms of the modified catalysts. In our calculations, we found concerted mechanisms for the oxidative additions studied. Our results show that the activation parameters of the oxidative additions have a clear correlation to the ligand type and ligand size. With PCl 3 and PBr 3 , our results show lower activation parameters than for the experimentally observed cis -[Rh(CO) 2 I 2 ]  . # 2002 Elsevier Science B.V. All rights reserved. Keywords: Rhodium; Density functional theory; Carbonylation; Catalysis; Oxidative addition; Phosphine 1. Introduction The Monsanto and Cativa processes, employing the catalytic carbonylation of methanol by rhodium or iridium catalysts, are one of the most effective proce- dures to manufacture acetic acid by industrial means. The basic reaction mechanisms and structures involved in the catalysis cycles have been studied for a long time to understand the system as a whole and to find improvement possibilities for the processes. There are both experimental [1  /5] and computational [6  /11] studies dealing with these systems. Part of the experi- mental studies have been concerned with the active catalytic species and the search for even better catalysts still continues. In our previous computational studies, we have explored the catalytic cycles of the Monsanto and Cativa processes systematically, studying also the species that have been so far unobserved experimentally [8  /10]. With our prior knowledge of these systems, we have taken a challenge to customize the actual active catalytic species and to test if some improvements could be found. This is the obvious advantage of the compu- tational chemistry; we are able to browse through vast series of different structures and see if any enhancements could be achieved. Our previous calculations suggest that the rate determining step of the Monsanto process could be accelerated by the experimentally unobserved trans - [Rh(CO) 2 I 2 ]  [9] but the trans -form is relatively more unstable than the corresponding cis -form. This study has shown a clear difference in the molecular orbital interactions while comparing the oxidative addition to cis -[Rh(CO) 2 I 2 ]  or trans -[Rh(CO) 2 I 2 ]  . The experi- mentally verified key feature of an efficient carbonyla- tion catalyst is a presence of electron-donor ligands. These lead to the idea of ligand modification, so that the active species should obtain the trans -form with the electron-donors present. The use of phosphines instead of iodides could provide the electron donating capability and proper steric effects to favor the trans -form of the active species. The idea of using phosphines in the carbonylation catalyst is not a new one [12] but a simple and systematic computational study will provide a new aspects to the topic. Our goal here is to see if a rather * Corresponding author. Tel.:  /358-8-5531681; fax:  /358-8- 5531603 E-mail addresses: tapani.kinnunen@oulu.fi (T. Kinnunen), kari.laasonen@oulu.fi (K. Laasonen). Journal of Organometallic Chemistry 665 (2003) 150  /155 www.elsevier.com/locate/jorganchem 0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0022-328X(02)02107-1