ELECTRON-TRANSFER BARRIERS FOR GROUND-STATE AND EXCITED-STATE REDOX COUPLES - ,11-TETRAMETHYL-1,4,8,11-TETRAAZACYCLOTETRADECANE) OSMIUM(VI)OSMIUM(V)/

Pulse radiolysis methods were used to measure rate and equilibrium con stants for the reaction of the Os-VI(TMC)(O)(2)(2+)/Os-V(TMC)(O)(2)(+) couple with semiquinone/quinone couples, Os-VI(TMC)(O)(2)(2+) + Q(-)r eversible arrow Os-V(TMC)(O)(2)(+) + Q, where TMC = tetramethylcyclam and Q is 1,4-benzoquinone or 2-methyl-1,4-benzoquinone. This work yiel ds a self-exchange rate constant of 1.1 x 10(6) M(-1) s(-1) and a redu ction potential of +0.048 V versus NHE for the dioxoosmium ground-stat e couple at 0.1 M ionic strength and 25 degrees C. [Os(TMC)(O)(2)](PF6 )(2) crystallizes in the monoclinic space group, P2(1)/c, with a = 6.7 12(2) Angstrom, b = 17.756(6) Angstrom, c = 10.150(2) Angstrom, beta = 95.59(2)degrees and Z = 2. Single crystal X-ray diffraction results f or the PF6- salt of the Os-VI(TMC)(O)(2)(2+) indicate that the dominan t isomer present in the crystal is the RSSR isomer in which one pair o f N-methyl groups is 'up' while the other is 'down'. The average Os-O and Os-N distances are 1.735(6) and 2.126(8) Angstrom, respectively.