The catalytic dehydrocoupling reactions of amine-boranes have attracted considerable interest from the perspectives of chemical hydrogen storage and the production of new inorganic polymers. In this review, significant progress made in coordination of amine-boranes at transition metals is described, and the elementary reactions in relation to the catalyzed dehydrocoupling process are also discussed. Highly unsaturated complexes as the precursors provide an efficient strategy to synthesize the complexes with amine-borane ligands. The amine-borane s-complexes with the h¹- or h²-coordination modes consist of electron-donation from the B-H bond to the metals exclusively and negligible back-donation to the B-H bond. The B-H activation (oxidative addition) of amine-boranes is achieved to the unsaturated Rh-PR₃ system, giving unique dimeric complexes with Me₃N-stabilized boryl ligands. The iridium amine-borane s-complexes convert thermally to the complexes with amino-borane ligands, together with elimination of hydrogen. X-ray crystallographic results of the ruthenium amino-borane s-complexes display shortening of the Ru-B and B-N bonds, indicating the B-N multiple-bond character. Dehydrocoupling reactions of primary and secondary amine-boranes promoted by a cationic iridium complex produce the linear diborazanes as kinetically stable products. The reaction pathway of the dehydrogenative oligomerization of amino-boranes is proposed.