Thioether complexes with the formula Delta/Lambda -chloro(thioether)bis(2,2'-bipyridine)metal(II) (M = Ru, Os; thioether = dimethyl sulfide (3a(+)), diethyl sulfide (3b(+)), and tetrahydrothiophene (3c(+))) have been synthesized. The rates of inversion at the sulfur atom of the thioether ligands have been measured by spin-inversion transfer and lineshape NMR methods. In every case, the ruthenium derivative exhibits a faster inversion frequency at a given temperature than the corresponding osmium derivative. In contrast, similar complexes with the formula chloro(delta/lambda -1 ,1'-biisoquinoline)(2,2':6',2 " -terpyridine)metal(II), 4(M=Ru,Os)(+), undergo atropisomerization of the misdirected 1,1'-biisoquinoline (1,1'-biiq) ligand with rates that are faster for osmium than ruthenium. As a result of the lanthanide contraction effect and the similar metric parameters associated with the structures of second-row and third-row transition metal derivatives, steric factors associated with the isomerizations are presumably similar for the Ru and Os derivatives of these compounds. Since third-row transition metal complexes tend to have larger bond dissociation enthalpies (BDE) than their second-row congeners, we conclude the difference in reactivities of 3(M=Ru)(+) versus 3(M=Os)(+) and 4(M=Ru)(+) versus 4(M=Os)(+) are attributed to electronic effects. For 3, the S3p lone pair of the thioether, the principal sigma donor orbital, is orthogonal to the metal a acceptor orbital in the transition state of inversion at sulfur and the S 3s orbital is an ineffective sigma donor. Thus, a regular relationship between the kinetic and thermodynamic stabilities of 3(M=Ru)(+) and 3(M=Os)(+) is observed for the directed reversible arrow [misdirected](double dagger) reversible arrow directed (DMD) isomerization (the more thermodynamically stable bond is less reactive). In contrast, atropisomerization of 4(+) involves redirecting (strengthening) the M-N bonds of the misdirected 1,1'-biiq ligand in the transition state. Therefore, an inverse relationship between the kinetic and thermodynamic stabilities of 4(M=Ru)+ and 4(M=Os)+ is observed for the misdirected reversible arrow [directed](double dagger) reversible arrow misdirected (MDM) isomerization (the more thermodynamically stable bond is more reactive). The rates obtained for 4(+) are consistent with the rates of atropisomerization of Delta/Lambda-(delta/lambda -1,1'-biisoquinoline)bis(2,2'-bipyridine)metal(II), 1(M=Ru,Os)(2+), and (eta (6)-benzene) Delta/Lambda-(delta/lambda -1,1'-biisoquinoline)halometal(II), 2(M=Ru,Os;halo=CI,I)(+), that we reported previously. We term the relative rates of reaction of second-row versus third-row transition metal derivatives kinetic element effects (KEE = k(second)/k(third)). While the KEE appears to be generally useful when comparing reactions of isostructural species (e.g. the relative rates of 1(M=Ru)(2+), 1(M=Os)(2+), and 1(M=Ir)(3+)), different temperature dependencies of reactions prevent the comparison of related reactions between species that have different structures (e.g., the 1,1'-biiq atropisomerization reactions of 1(M=Ru,Os)(2+) versus 2(M=Ru,Os;halo=CI,I)(+) versus 4(M=Ru,Os)(+)). This problem is overcome by comparing entropies of activation and kinetic enthalpy effects (KHE = DeltaH(third)(double dagger)/DeltaH(second)(double dagger)). For a given class of 1,1'-biiq complexes, we observe a structure/reactivity relationship between DeltaH(double dagger) and the torsional twist of the 1,1'-biiq ligands that are measured in the solid state.