The Effect of Ionic Strength and Pressure on the Complex Formation and Hydrolysis Equilibria in the Aquated Iron(III)‐Thiocyanate System

Abstract

The equilibrium constants for the complex formation of Fe(H2O)3+6 with SCN− (K1) and the hydrolysis of Fe(H2O)5SCN2+ to produce Fe(H2O)4(OH)SCN+ (K4) were studied as a function of ionic strength (0.1 to 1.5 M) at 25°C and pressures up to 120 MPa. The ionic strength dependencies at ambient pressure can be expressed as log K1 = 2.86 ‐ 3.05 √ μ (1 + 2.96 √ μ)−1 + 0.04 μ and log K4 = ‐0.93 ‐ 1.02 √ μ (1 + 1.97 √ μ)−1 + 0.06 μ, from which the distance of closest approach follows to be 0.9 and 0.6 nm, respectively. The pressure dependencies of K1 and K4 result in reaction volumes ranging from 10.0 to 7.5 and 11.9 to 11.0 cm3 mol−1, respectively, for zero to 1.5 M ionic strength. These data combined with those for the hydrolysis of Fe(H2O)3+6 reported before, enable the estimation of the equilibrium constant (K3) for the formation of Fe(H2O)4(OH)SCN+ from Fe(H2O)5OH2+ and SCN−, as a function of ionic strength and pressure. This is the first study to report such data for K3, and K4, and the results are discussed in reference to data for the overall system.

Electron Transfer. 147. Reductions with Gallium(I)1

“…An [NCS - ]-proportional term in the latter, attributable to a NCS - -bridged component, has disappeared, and an inverse-[H + ] term, almost certainly related to deprotonation of an Fe(III) center, has become prominent. Within the pH range examined, the preponderant Fe(III) species is [Fe(H 2 O) 4 (OH)(NCS)] + (rather than Fe(H 2 O) 5 (NCS) 2+ ; p K A −0.8) . We are then dealing with the preequilibrium The bimolecular rate constant associated with the active dihydroxo complex, obtained by dividing k - 1 by 10 -3 (an estimated K A ), falls near 6 × 10 5 M -1 s -1 , a value about 10 2 times that for the [H + ]-independent rate (7 × 10 3 M -1 s -1 ).…”