The synthesis and properties of dihydrogen complexestrans-[MH(H2)L]+, M = Fe, Ru, Os, which contain the ligandmeso-tetraphos-1,S,R-Ph2PCH2CH2P(Ph)CH2CH2P(Ph)CH2CH2PPh2(L) are described. There are interesting possibilities of isomerism in suchtranscomplexes because the axial binding sites at the metal are different, one being surrounded by four phenyl groups and the other by two phenyl groups. The osmium complex is prepared in an unusual reaction ofcis-β-Os(Cl)2L with H2(1 atm) and NaBPh4(1 mol) in THF or by the reaction oftrans-OsH(Cl)L with NaBPh4and H2. The iron and ruthenium complexes were made by a reaction of HBF4with complexestrans-M(H)2L that have inequivalenttranshydrides. The ruthenium complex was also prepared starting from isomers oftrans-RuH(Cl)L. The H—H distance in the rapidly spinning dihydrogen ligand has been calculated fromT1(min) data to be 0.88, 0.89, and 0.99 Å for the complexes of Fe, Ru, Os, respectively. The presence of the H—D bond in the isotopomerstrans-[MH(HD)L]+andtrans-[MD(HD)L]+is also confirmed by the observation of1JHDcoupling constants of 32, 33.5, and 26.4 Hz for Fe, Ru, and Os, respectively. There is no rapid intramolecular H atom exchange in these complexes in contrast to those with di-tert-phosphine ligands like [MH(H2)(dppe)2]+or to the trihydride Re(H)3L. Described also are the properties of the precursor complexes includingcis-β- andtrans-Ru(Cl)2L and derivatives of the dihydrogen complexestrans-[MH(L′)L]+, L′ = CH3CN (on Ru and Os), PMe2Ph (on Ru), and CO (on Os). Trends in the NMR properties of isostructural complexes are reported.