The logarithm of the pseudo-first-order rate coefficient k for the aquation of CO(NH,),X'~-"'+ can be represented by a quadratic equation in the pressure P, or, better, bywhere Pis in kbar, AV,* is the volume of activation at P = 0, and xis the increase in the number of water molecules solvating the complex as it goes to the transition state. For Xn-= SO:-, Cl-, NO;, and N; at 25' (75' for N;) and ionic strength I = 0.1 M LiC104/HC104, A G = -18.5, -10.6, -9.2, -6.3, and + 16.8 cm3 mol-', and x = 8.0,4.1,3.9, 1.9, and -4.2; for X"-= NCS-, the mean A V* from P = 0.001 to 2.5 kbar at 88" is -4 cm3 mol-'. Detailed consideration of these data, especially their correlation with the molar volume of reaction by a straight line of unit slope for X"-= SO:-, C1-, Br-, NO;, and H,O, provides strong evidence for a dissociative interchange mechanism. For Xn-= N;, the separating entity is probably HN, rather than N;. For Xn-= NCS-, aquation is incomplete, at practical complex concentrations; at 88.0°, 1 bar, and I =

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