Transition metal clusters are molecules containing three or more metal atoms that are connected by direct metal-metal bonds. Ever since the cluster-surface analogy was proposed, the utility of clusters in the catalytic transformation of substrates has been explored with considerable interest. A large number of studies on the reaction chemistry of clusters have been reported, to date using various types of clusters. Most of the metal clusters used in these studies have been binary carbonylmetal complexes or clusters directly derived from them. However, from the results of these studies, it could not be inferred that the newly developed reactions were characteristic of multimetallic systems.

On the basis of the idea that multiple metal sites will participate in reactant-to-product transformation if the metal cluster spontaneously causes vacant coordination sites at the adjacent metal centers, Prof. Suzuki began his project on reaction chemistry of transition metal polyhydride clusters in the mid-1980s. Although several examples of multinuclear polyhydride-bridged complexes having phosphanes as auxiliary ligands were previously reported, no polyhydride clusters having only cyclopentadienyl ligands as auxiliaries were known until Prof. Suzuki discovered dinuclear ruthenium tetrahydride, (C₅Me₅)Ru(μ-H)₄Ru(C₅Me₅). His new approach, namely, systematic studies on the synthesis of the fascinating noncarbonyl cyclopentadienyl ruthenium polyhydride-bridged clusters and in-depth analysis of associated reactions has resulted in tremendous progress being made in the field of reaction chemistry of clusters.

1. Synthesis of transition metal polyhydride clusters having C₅Me₅ groups
The electron density at the metal centers of metal polyhydride clusters is higher than that at the metal centers of conventional polycarbonylmetal or polycarbonylmetal hydride clusters. Owing to the higher electron density at the metal centers, metal polyhydride clusters are expected to be highly active toward oxidative addition of reactants. In addition, a metal polyhydride cluster is considered to be a suitable and versatile precursor of the active species for multimetallic activation. The liberation of bridging hydride ligands from the metal cluster gives rise to vacant coordination sites on the adjacent metal centers, which might cooperate with each other to activate the reactant as a binding site and an activation site. Furthermore, the introduction of C₅Me₅ groups into the cluster framework increases the electron density at the metal centers.
In 1988, Prof. Suzuki synthesized the first dinuclear tetrahydride-bridged ruthenium complex, (C₅Me₅)Ru(μ-H)₄Ru(C₅Me₅), having only cyclopentadienyls as auxiliary ligands and showed that each metal center took the role of both a binding site and an activation site and that the metal centers cooperated with each other to activate the substrate. Then, he carried out systematic researches on the following three subjects: (1) development of synthetic methods for polyhydride clusters with high nuclearity ranging from three to six, (2) modification of the electronic and steric circumstances of the reaction sites by introducing different heteroatoms or heteroatom groups into the core of the cluster as a bridging ligand, and (3) development of synthetic methods for mixed-metal polyhydride clusters containing ruthenium and a series of transition metals from groups 4-9. On the basis of these researches, he established rational synthetic methods for a large number of transition metal polyhydride-bridged clusters with various combinations of nuclearity, auxiliary ligands, number of hydrido ligands, formal charge, etc. These structural factors are correlated with parameters pertaining to the reactivity of the clusters, such as redox potentials, acidity/basicity of hydride ligands, and free energy of activation for the exchange of coordination site of the hydride ligands.

2. Multimetallic activation of hydrocarbon molecules
The term �gmultimetallic activation�h refers to the activation of a substrate by the cooperative action of multiple metal centers. The newly developed cyclopentadienylmetal polyhydride system brought about several important changes in the reactivity of the clusters because metal centers are tightly bound to each other by multiple bridging hydride ligands. The above-mentioned metal polyhydride clusters exhibit remarkable reactivity toward hydrocarbons including alkanes, ammonia, and carbon dioxide. Prof. Suzuki has realized many unprecedented modes of reactions, such as cleavage of chemically non-activated C-C bonds, consecutive cleavage of C-H bonds of alkane, and selective recombination of hydrocarbyl ligands on the trinuruthenium cluster as a result of consecutive redistribution concomitant with cleavage and reforming of the Ru-Ru bond. The results demonstrate the potential applicability of cluster compounds in synthetic reactions and shed light on the mechanism of reactions on metal surfaces.
In addition, his achievements partly cover surface science. The study of the structure and chemical behavior of trinuclear clusters with face-capping benzene, pyridine, and triangularly arranged carbene ligands is considered to be appropriate for analyzing adsorbents on metal surfaces.

As described above, Prof. Suzuki has succeeded in the systematic synthesis of transition metal polyhydride clusters with various nuclearities and established a new field of study called �greaction chemistry of transition metal polyhydride clusters.�h Such pioneering work stems from his original ideas, and he is highly recognized in the academic world for his contribution to the field of metal cluster chemistry.