Tabulating the structures and characteristic NMR properties of 17 iron complexes, 98 ruthenium complexes and 70 osmium complexes that contain dihydrogen or compressed dihydride ligands reveals a variety of trends. The H H bond lengths increase from similar Fe(II) to Ru(II) to Os(II) complexes. Iron(II) displays a narrow range of H H distances for stable complexes. Electronegative atoms Cl and O, when attached on the metal trans to the dihydrogen ligand, result in elongation of the H H bond relative to more electropositive atoms H, C, P and N. The family of cyclopentadienyl ligands also causes this elongating effect. The dihydrogen ligands with short H H distances and weak interactions with the metal, especially on iron and ruthenium are in the fast spinning regime. One exception is the biporphyrin complex of ruthenium with the side-on bridging H 2 ligand which has an H H distance of 118 pm but is in the fast spinning regime. There are some ruthenium complexes with H H distances greater than 110 pm that are in the slow motion regime and several complexes of osmium with H H distances greater than 130 pm that Abbreviations: binap, 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl; bpy, bipyridine; bpzm, bis(pyrazol-1-yl)methane; cod, 1,5-cyclooctadiene; dach, trans-1,2-diaminocyclohexane; dcpp, PCy 2 CH 2 CH 2 CH 2 PCy 2 ; dcype, PCy 2 CH 2 CH 2 PCy 2 ; DFT, density functional theory; dppip, 2,2-bis(diphenylphosphino)propane; diphos, chelating bis(diphosphino)ligand; dmpe, PMe 2 CH 2 CH 2 PMe 2 ; dmpm, PMe 2 CH 2 PMe 2 ; dmpm, bis-dimethylphosphinomethane; dpen, 1,2diphenylethylenediamine; dippach, P i Pr 2 NHC 6 H 10 NHP i Pr 2 ; dippae, P i Pr 2 NHCH 2 CH 2 NHP i Pr 2 ; dppe, PPh 2 CH 2 CH 2 PPh 2 ; dppp, PPh 2 CH 2 CH 2 CH 2 PPh 2 ; dppm, PPh 2 CH 2 PPh 2 ; HBPz 3 -, hydridotris(pyrazolyl)borate; HCn, 1,4,7-triazacyclononane; imid * , 1-tert-butyl-3-phenyl-imidazole; INS, inelastic neutron scattering; MeCN, 1,4,7-trimethyl-1,4,7-triazacyclononane; meso-tetraphos-1, PPh 2 CH 2 CH 2 PPhCH 2 CH 2 PPhCH 2 CH 2 PPh 2 ; oep, octaethylporphyrin; OTf -, CF 3 SO 3 -; pcp, [C 6 H 3 (CH 2 PPh 2 ) 2 -2,6] -; Pcyp 3 , tris(cyclopentyl)phosphine; pybuS 4 2-, 2,6-{2,5-t Bu-6-S --C 6 H 2 SCH 2 } 2 C 5 H 3 N; Spy -, 2-mercaptopyridine anion; QEC, quantum mechanical exchange coupling; tpm, tris(pyrazolyl)methane.