Kinetic studies on the dimerization of molybdenum(V) in acid media and the crystal structure of K[MoOCl4(OH2)] · H2O

[MoOCl₅]^2- aquates rapidly in solutions ofca. 5 M or less in HCl or MeSO₃H to give an intermediate which changes to the well known ion, Mo₂O_4aq²⁺ at a lower rate. Solutions of molybdenum(V) in 12M MeSO₃H, when diluted, also change at a similar rate to Mo₂O₄²⁺. The rate of reaction, d[Mo₂O₄²⁺]/ dt=k[Mo^V] has a value of k=30.(1)s^-1 at 0°C in 5.00 MMeSO₃H. At constant ionic strength, k is nearly independent of acidity (5.2–0.5 M) and is only slightly affected by exchanging MeSO₃^- with Cl^- orp-toluenesulfonate ion. At low ionic strength k increases dramatically. In 4.78 MMeSO₃H, the activation parameters are: ?H^?=72.68(29) kJ/mole, ?S^?=16.63(8) jmole · K and k=5.01(5)×10²s^-1 at 0°C.¹⁸O measurements on the oxygen transfer between MoO_aq³⁺ and Mo₂O_4aq.²⁺ were partially inconclusive. A mechanistic interpretation is given to the kinetic results. The x-ray crystal structure of KMoOCl₄(OH₂)·H₂O is reported.