MESP: An efficient method to validate an ONIOM partition for the modelization of phosphine ligands commonly used in the Pauson-Khand reaction

In this study we discuss the modelization of phosphine ligands commonly used in inorganic and organometallic chemistry as for example the asymmetric Pauson-Khand reaction by means of ONIOM method. A fundamental question that arises after the decision of using QM/MM methods for a certain study is how to partition the system. Which atoms are going to be included in the QM region and which atoms are going to be included in the MM region? Chemical knowledge is most of the time the guiding line for this choice, but tests should be run in case of uncertainty. For the setting of the QM/MM border, two criteria have been employed: results from geometry optimizations of the ligand complexed to tungsten penta(tetra)carbonyl and calculations of the molecular electrostatic potential (MESP) for the ligand alone. We have shown that these two methods give similar conclusions and thus MESP can be used to validate the ONIOM partition of these systems. This later approach has the advantage of being fast and computationally inexpensive.