Monodentate and bidentate phosphine supported transition metal-dihydrogen complexes were studied extensively in last three decades by using spectroscopic (IR, NMR) and structural (X-ray and neutron single crystals diffraction) characterization techniques. A major concern was to investigate various changes in physical and chemical properties of H₂ when bound to a metal center. This article emphasizes the role of the electron rich phosphin ligands in forming molecular hydrogen complexes for a large number of transition metal ions and how the bonding of the metal with the molecular hydrogen (H₂) affects the ultimate fate of the H−H bond. Important features of the H₂ complexes such as elongation of the H−H bond, rapid dynamics of the site exchange of H atoms in hydride/dihydrogen complexes, binding of H₂ to the coordinately unsaturated metal centers are discussed in great detail. In many cases, the results of spectroscopic and structural studies were supported by theoretical calculations. Besides these, the focus of the present article is on the phenomena of reversible release of H₂ observed in the case of certain recently reported metal-H₂ complexes which has relevance in H₂ production. Various features of the elongated H₂ complexes and reversible binding of H₂ to a metal center discussed in this article will be of great interest to inorganic and organometallic chemists in general and to inorganic-material chemists working in the area of catalysis and storage materials using H₂. Discussions on certain new directions of metal-dihydrogen chemistry such as photochemical generation of H₂ complexes, ionic H₂ activiation, and H₂ reductive elimination from polyhydride complexes have been included (202references).