Theoretical and Computational Study of a Complex System Consisting of Transition Metal Element(s): How to Understand and Predict Its Geometry, Bonding Nature, Molecular Property, and Reaction Behavior

Abstract

Abstract Complex chemical systems consisting of transition metal element(s) are important and attractive research targets in both experimental chemistry and theoretical chemistry. Transition-metal complexes with carbon dioxide are discussed as one example, in which the coordination geometry, bonding nature, and reactivity are understood well and predicted with the HOMO and the number of d electrons. The spin-multiplicity of inverse sandwich-type dinuclear transition-metal complexes, which have been synthesized recently as a new type of compound, is discussed as another example based on CASPT2 calculations, in which the clear relationship between the spin multiplicity of its ground state and the number of d electrons is presented. Theoretical understanding of various organometallic reactions has been an important endeavor over the last two decades. Insertion reactions of olefin and carbon dioxide into M–H and M–alkyl bonds and σ-bond activation reactions are discussed with orbital interaction diagrams based on perturbation theory. In particular, detailed discussion of the characteristic features of a new type of oxidative addition to an M–L moiety (L = neutral ligand such as alkene and alkyne) and heterolytic σ-bond activation by an M–X moiety (X = anionic ligand). It is still a central challenge to elucidate the reaction mechanisms of catalytic reactions by transition metal complexes. The reaction mechanisms and electronic processes of Ru-catalyzed hydrogenation of carbon dioxide, Pt-, Rh-, and Zr-catalyzed hydrosilylations of alkene, Ir-catalyzed borylation of benzene, and the Hiyama cross-coupling reaction are analyzed based on computational results. We wish to present how to understand the mechanism based on the number of d electrons and the energy of d orbitals in discussion. Also, the importance of solvation and crystalline effects in the theoretical study of transition-metal complexes is discussed based on our recent theoretical studies of mixed-valence complex and a single crystal of a Pt(II) complex.