The synthesis and characterization of several pentaamminecobalt(III) complexes of 5-substituted tetrazoles are reported: 5-cyanotetrazole, 5-methyltetrazole, 5-carbamoyltetrazole, and unsubstituted tetrazole. Characterization by proton, carbon-13, and nitrogen-15 NMR and by ultraviolet-visible spectroscopy established that bonding between cobalt(III) and the tetrazole ring occurs at the nitrogen at ring position 2 in agreement with previous crystal structure determinations on the 5-methyl and 5-cyanotetrazole complexes. Kinetics of the hexaaquachromium(II) reduction of these various cobalt(III) complexes is reported as well as the reduction kinetics for the N-1-bonded (5-methyltetrazolato)pentaamminecobalt(III) complex, the linkage isomer of the N-2-bonded 5-methyl complex. All reductions proceed principally by an inner-sphere electron-transfer process with reduction pathways appearing for both the protonated (3+) and deprotonated (2+) complexes characterized by rate constants k0 and k1, respectively. A mechanistic scheme common to all five complexes is proposed that predicts the operation of two limiting forms of the rate law governed by the pKa's of the five complexes. At 25°C, k0 values are 19 ± 2, 0.15 ± 0.08, and 2.4 ± 0.3 L mol-1 s-1 for the tetrazole, N-2-bonded 5-methyltetrazole, and N-1-bonded 5-methyltetrazole complexes, respectively. The k1 values are 3.3 ± 0.3, 0.41 ± 0.02, and 0.83 ± 0.02 s-1 for these same three complexes at 25°C. Separation of the observed rate constants into the two components was not possible for the 5-cyanotetrazole and 5-carbamoyltetrazole complexes.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry