The preparation and spectroscopic properties are described of some platinum(II) complexes having a hydride ligand cis or trans to an sp 3 carbon, viz. trans-PtH(YCN)(PPh 3) 2 and cis-PtH(YCN)(LL) with YCN = C 2H 4CN, n-C 3H 6CN, o-CH 2C 6H 4CN and LL = bis(diphenylphosphino)-ethene or -ethane. The complexes trans-PtH(YCN)(PPh 3) 2 can add a fifth ligand in solution; the resulting five-coordinate complex was observed by 31P NMR in the case of PtH(C 3H 6CN)(PPh 3) 3. Insertion of olefin (ethen, 1-cyanoethene, norbornadiene, allen) into the PtH bond of the trans-hydrido complexes occurs to give cis-dialkyl complexes, but the cis-hydrido complexes are unreactive. The mechanism of insertion is discussed in terms of the kinetics and the geometries of reactants and products.