The nitrito-nitro linkage isomerisation reaction of the penta-amminecobah( 111) complex in liquid ammonia proceeds entirely via an intramolecular conjugate-base mechanism. The reaction is fully retentive and the penta-ammine(nitro)cobalt(iii) complex is the only species that could be detected by H n.m.r. after the reaction was completed. The advantage of liquid ammonia as solvent is demonstrated by the separate determination of the pre-equilibrium constant P a b -(AH = 27 kJ m0l-l ; AS = -14 J K-' mol-l) and the rate constant of the rate-determining step k2
The position of the Mo atom was determined by direct methods. All the remaining non-hydrogen atoms were located by structure factor calculations and difference electron density maps. The structure was refined by full-matrix least-square techniques, minimizing the function E*v(|F0| -|FC|)1 2 with w = 4F02 4/[V(F02) + (pF02)2], where p was set equal to 0.03. Hydrogen atoms were included as fixed contributions at idealized coordinates in the final cycles of refinement. Refinement of the model with anisotropic thermal parameters for Mo, NI, N2, SI, S2, 01, and 02 resulted in Rx = || 0| -|FC||/£|F0| = 0.074 and R2 = [X>(|F0| -|Fc|)2/EwF02]V2 = 0.058. The "goodness of fit" E>v(|F0| -|Fc|)2/(nm)]1/2 = 1.569, where n is the number of data (2355) and m is the number of variables (204). Atomic coordinates are given in Table V; anisotropic thermal parameters and values of |F0| and |FC| are available as supplementary data.
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