The preparation, structure, dynamic behavior in solution, and reactivity of polyhydride complexes of platinum group metals, described during the last three decades, are contextualized from both organometallic and coordination chemistry points of view. These compounds, which contain dihydrogen, elongated dihydrogen, compressed dihydride, and classical dihydride ligands promote the activation of B-H, C-H, Si-H, N-H, O-H, C-C, C-N, and C-F, among other σ-bonds. In this review, it is shown that, unlike other more mature areas, the chemistry of polyhydrides offers new exciting conceptual challenges and at the same time the possibility of interacting with other fields including the conversion and storage of regenerative energy, organic synthetic chemistry, drug design, and material science. This wide range of possible interactions foresees promising advances in the near future.

The synthesis, characterization, and ethylene polymerization behavior of a family of chromium(III) complexes of formula [2,6-bis(imino)pyridyl]CrCl3 is reported. The X-ray diffraction studies of two of the new compounds show that the geometry around the chromium atom is octahedral, with the three chlorine ligands in a mer disposition. The distance between the metal and one of the trans-disposed chlorine atoms is significantly longer than the other two Cr−Cl distances. Treatment of the complexes [2,6-bis(imino)pyridyl]CrCl3 with methylaluminoxane (MAO) leads to very active ethylene polymerization catalysts that afford highly linear polyethylene. The substituents at the ortho position of the N-aryl groups of the 2,6-bis(imino)pyridyl ligands modulate both the catalytic activity and the molecular weights of the resulting polyethylene. The most active catalysts are those with two substituents at the ortho position of the N-aryl groups (activities up to 4 × 107 g (mol of Cr)-1 bar-1 h-1 are achieved). Regarding the size of the substituents, the activity and the molecular weights follow an opposite trend. Systems with two small substituents lead to very active systems, but the molecular weight of the polyethylene is lower than when bulkier substituents are present.

An easy and direct access to POP-type osmium(II) and osmium(IV) complexes, including OsH 4 {dbf(P i Pr 2 ) 2 } (dbf(P i Pr 2 ) 2 = 4,6bis(diisopropylphosphine)dibenzofuran), is reported. This tetrahydride derivative is an efficient catalyst for the selective formation of imines from alcohols and amines with liberation of H 2 , proving that osmium is a promising alternative to ruthenium for catalysis.