Enforced slow protonation of [Fe4S4Cl4](2-) and the maximum rate of protonation of the cluster core

Kinetic studies on the reaction of PhS- with [Fe4S4Cl4](2-) (pK(a)=18.8) in the presence of the weak acid [NH2(CH2)(3)CH2](+) in MeCN showed that the mechanism involves initial, rate-limiting, binding of PhS- to the cluster, followed by protonation of the cluster core (presumably a mu(3)-S), then di ssociation of chloride. This is different to the sequence of elementary rea ctions established with the stronger acid, [NHEt3](+)(pK(a)=18.5), where pr otonation precedes binding of the thiol. Quantitative comparison of these t wo systems reveals that the literature value of pK(a)=19.6 for [NH2(CH2)(3) CH2](+) is inconsistent with our kinetic results and thatpK(a)=21.5 is more appropriate, both in this and other systems. The kinetic data show that th e rate of protonation of the cluster core falls in the range 2x10(5) less t han or equal to k less than or equal to 4.8x10(6) dm(3) mol(-1) s(-1), for a thermodynamically favourable reaction. The reasons why this protonation i s 10(4)-10(5) times slower than the diffusion-controlled limit are discusse d.