trans-[Os(tpy)(Cl)(2)(N)]Cl (1) has been prepared by addition of tpy to [NBu(4)][Os(N)Cl-4] in dichloromethane-and trans-[Os(tpy)(Cl)(2)(N)][PF6] (2) by addition of [NH4][PF6] to a methanolic solution of 1. When dissolved in methanol in the presence of added chloride, 1 undergoes slow isomerization to cis-[Os(tpy)(Cl)(2)(N)](+), which has been isolated as its [PF6](-) salt 3. In methanolic solutions containing added chloride, an equilibrium is set up between the trans and cis isomers, trans-[(tpy)(Cl)(2)Os=N](+) reversible arrow cis-[(tpy)(Cl)(2)Os=N](+) with K(20 degrees C) = k(f)/k(r) = 4.8, Delta H degrees = 22(2) kJ/mol, and Delta S degrees = 88(6) J/mol as measured by H-1 NMR. The approach to equilibrium followed first order kinetics as determined by time-resolved UV-visible measurements with k(f)(20 degrees C) = 1.1 x 10(-4) s(-1), k(r)(20 degrees C) = 2.3 x 10(-5) s(-1), Delta H-f(double dagger) = 78(8) kJ/mol, Delta S-f(double dagger) = 79(10) J/mol, Delta H-r(double dagger) = 56(7) kJ/mol, and Delta S-r(double dagger) = -9(6) J/mol. In the absence of added chloride, rapid solvolysis occurs to give trans- or cis-[Os(tpy)(Cl)(MeOH)(N)](2+) as shown by absorption and conductivity measurements. Isomerization does not occur in dichloromethane. A mechanism for isomerization and substitution is proposed involving associative attack of methanol to form seven-coordinate, solvent-bound intermediates which undergo solvolysis by loss of chloride or isomerization by interchange of the chloro and nitrido groups.