# Mechanism and Steric Course of Octahedral Aquation. XIV. The Activation Parameters and Steric Course of Aquation of trans-Chloro- an trans- Bromoazidobis(ethylenediamine)cobalt(III) Cations

Ricevuto, Vittorio and Tobe, Martin L. (2005) Mechanism and Steric Course of Octahedral Aquation. XIV. The Activation Parameters and Steric Course of Aquation of trans-Chloro- an trans- Bromoazidobis(ethylenediamine)cobalt(III) Cations. Accademia Peloritana dei Pericolanti Classe FF.MM:NN., 83. pp. 59-65.

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atti_3_suppl.n.2_2005_59.pdf

The rates and steric course of aquation of $_t_r_a_n_s-[Co(en)_2N_3Cl]ClO_4$ have been reexamined over a 50° range of temperature. At 25.0°, $[H^+]=0.01M,_k=2.6*10^-^4_s_e_c^-^1,\Delta H =20.7 \pm \ 0.3 kcal/mol, and \Delta S= -6 \pm \ 2 eu.$ The corresponding bromo complex has been prepared and examined for the first time. At 25.0°, $[H^+]=0.01 M,_k=8.5*10^-^4_s_e_c^-^1, \Delta H = 20.0 \pm \ 0.5 kcal/mol, \Delta S= -6 \pm \ 2 eu.$ In all cases, after correction for subsequent isomerization, it is shown that aquation occurs with complete retention of configuration. The specific rate of approach to equilibrium of $_c_i_s- and _t_r_a_n_s- Co(en)_2N_3OH^2^+$ was also measured over a 30° temperature range. At 25.0°, $[H^+]=0.010 M,_k=9.6 * 10^-^5 _s_e_c^1-^1, \Delta H= 32.0 \pm \ 0.5 kcal/mol, \Delta S= +30 \pm \ 3eu.$ The equilibrium composition (43% $_t_r_a_n_s$) is independent of temperature over this range. The steric course of spontaneous and $H_g^2^+$ -catalyzed aquation of $_c_i_s-Co(en)_2N_3Cl_+$ has also been rexamined. It has been shown conclusively that both process give identical amounts of the $_c_i_s-$ and $_t_r_a_n_s-$aquoazido products and there is reasonable evidence to suggest that some 15% of the intial reaction product has the $_t_r_a_n_s$ configuration- This is the first example of stereochemical change in the spontaneous aquation of a $_c_i_s-Co(en)_2AX^n^+$complex.