KINETIC AND MECHANISTIC STUDY OF THE LINKAGE ISOMERIZATION OF SUBSTITUTED (5-PHENYLTETRAZOLATO)PENTAAMMINECOBALT(III) COMPLEXES

The facile synthesis of a series of (5-(R-phenyl)tetrazolato)pentaamminecobalt(III) complexes which undergo a spontaneous linkage isomerization reaction from the Nl- to the N2-bonded linkage isomers is reported for R = 4-CH3, H, 4-C1, 3-CF3, and 4-N02. The first-order rate processes show a strong pH dependence with rate constants for the protonated species (k3+) approximately 50 times those of the corresponding deprotonated complexes (k2+). The rate constants and the acid equilibrium constants (Ka1) for the N1 complexes show a good correlation with the Hammett σ parameter, while the N2 acid equilibrium constants (Ka2, determined separately) correlate well with the aσ parameter. Kinetic and thermodynamic parameters (k3+, k2+, Ka1, and ka2) determined for each system at 25 °C and I = 1.0 M are 3.6 X 10−3, 4.85 ×10−5, 0.14, and 0.13; 4.8 × 10−3, 6.1 × 10−5, 0.26, and 0.16; 9.4 × 10−3, 1.40 × 10−4, 0.40, and 0.27; 2.1 × 10−2, 2.3 × 10−4, 1.1, and 0.62; and 3.1 × 10−2, 5.6 × 10−4, 1.8, and 2.2 for R = 4-CH3, H, 4-C1, 3-CF3, and 4-N02, respectively, where units of s−1for the rate constants and M for the acid dissociation constants are quoted. Relief of steric hindrance as a major driving force for the isomerization is consistent with the data presented. An empirical method for the determination of % N2-bonded species present in N1 preparations is presented. © 1990, American Chemical Society. All rights reserved.