One of the important challenging research targets in theoretical and computational chemistry has been transition metal complexes in the last two-three decades. Professor Shigeyoshi Sakaki has made a wide range of original contribution in this field. Although theoretical and computational study of transition metal complexes is more difficult than that of organic molecules, he started his theoretical work of organometallic reaction in the early 1970s, which was the first theoretical calculation of organometallic reaction, to our knowledge. Since then, he is continuing the theoretical and computational study of complex systems consisting of transition metal elements; in other words, he is one of the pioneers in this field. He successfully elucidated the origins of the flexible structures, chemical bonds, and reaction behaviors of transition metal complexes and provided theoretical predictions based on the electronic structures. It should be also noted that he made large contributions to the interplay between computational study and experiment. His research contributions are summarized, as follows:

Analysis and Computational Methods for Transition Metal Complex: It is not easy to extract essence and clear understanding from computational results of transition metal complexes because of their complicated and flexible electronic structures. Professor Sakaki proposed the orbital mixing rule based on the perturbation theory and successfully provided clear understanding of the electron distribution of transition metal complex and the electronic process of chemical reaction of transition metal complex. Another important contribution by him is the recent proposal of a new QM/MM method for molecular crystal. Though the infinite system is mainly investigated with the plane-wave DFT method now, it is not easy to employ high quality DFT functional and post-Hartree-Fock method in such calculation. In his QM/MM method, we can employ various functionals and post-Hartree-Fock method. He successfully optimized the position and the orientation of the lattice water in the single crystal of Pt(II) complex with his method. This is one of the new frontiers of molecular theory for the infinite system.

Geometries, Bonding Natures, and Electronic Structures of Transition Metal Complexes: Many transition metal complexes of carbon dioxide, dinitrogen molecule, dihydrogen molecule and heavy main-group element species were synthesized in the experimental chemistry. Their geometries, bonding natures, and properties could not be understood based on classical idea. Prof. Sakaki has made remarkable contributions to developing and enriching the chemistry of these complexes. He investigated a number of unique transition metal complexes of CO₂ and dinitrogen molecule as well as heavy main-group element species. He found a general rule between the d-orbital occupation and the coordination structure of CO₂, which provides well understanding and prediction of the transition metal complexes of CO₂. Also, his theoretical study of multi-nuclear transition metal complexes provided well understanding of their geometries and bonding natures based on the electronic structure. We wish to emphasize his state-of-the-art investigations of transition metal complexes with near-degeneracy, because the near-degeneracy is one of the important issues in transition metal complexes. Such works have been reported only by a few groups. He succeeded to elucidate the electronic process of the reaction between Fe(II) and O₂ molecule as well as the origins of surprisingly high spin multiplicity of inverse sandwich type complexes, based on the multi-reference wavefunction. His recent theoretical studies of metal-organic-framework (MOF) should be noted too, in which he succeeded to provide clear understanding of spin-transition of MOF and gas-absorption by MOF. These results have been widely accepted as important findings in the experimental chemistry.

Chemical Reactions by Organometallic Complexes: Prof. Sakaki is the first to perform the theoretical study of the organometallic reactions on 1974. Since then, he performed pioneering theoretical studies of almost all fundamentally important organometallic reactions. His theoretical work covers the nucleophilic and electrophilic attacks to π-electron systems coordinated with metal, the migratory insertion reactions of CO, C_O2, and ethylene, the 2+2 oxidative cyclization reactions, the oxidative addition/reductive elimination reactions of C-H, C-C, Si-H, Si-C, and Ph-CN bonds, and the heterolytic σ-bond activation reactions of H-H and C-H bonds. In particular, his study of heterolytic C-H σ-bond activation reaction should be emphasized here; he first pointed to this type of C-H activation which had not been recognized experimentally and theoretically before his work. He disclosed what is the driving force and what orbital interaction is important. These results impacted the experimental chemistry of transition metal complexes, as reviewed recently.

Catalytic Reactions by Transition Metal Complexes: As well known, transition metal complexes exhibit important catalyses in various chemical reactions. It is of considerable importance to elucidate the reaction mechanism and the origin of catalysis of transition metal complex for further development in this field. Prof. Sakaki has elucidated reaction mechanisms and electronic processes of many catalytic reactions by transition metal complexes. Though many DFT studies have been reported nowadays, important is to provide general understanding and essence of catalytic reaction. His theoretical studies of hydrosilylation of olefin elucidated the reaction mechanism and also disclosed that the reaction mechanism is determined by the number of d electrons in the late transition metal complex but the d orbital energy is important in the early transition metal complex. Though many theoretical studies have been recently performed on the CO₂ fixation, he started his theoretical studies of the CO₂ complexes and the CO₂ fixation reactions in the late 1980s and successfully elucidated the new reaction mechanism and the determining factor for reactivity. Recently, he made significantly important contributions to cross-coupling reactions. He elucidated why the reaction easily occurs and what is important to accelerate the transmetallation that is a key elementary step. Also, he reported theoretical prediction how to construct a catalytic cycle with a main-group element compound. His prediction is of considerable importance, because the catalytic cycles by main-group element compounds have been very limited in the experimental chemistry.
In summary, he made a lot of significant contributions to theoretical and computational studies of transition metal complexes which have been one of the challenging research targets. His fruitful researches have been of considerable importance in both of theoretical/computational chemistry and experimental chemistry.