Oxalic oxygen exchange with

Oxygen exchange with mixed chromium(m)-oxalate complexes [Cr(ox)(LL)2]+ has been described. ... 

A theoretical study of the thermal isomerization and decomposition of oxalic acid has attempted to account for the predominant formation of C02 and HCOOH from the vapour at 400-430 K.41 Transition-state theory calculations indicate that a bimolecular hydrogen migration from oxygen to carbon of intermediate dihydroxycarbene (formed along with C02) achieved through a hydrogen exchange with a second oxalic acid... 

While unequivocal determination of rearrangement pathways is usually impossible, experimental data can often exclude certain possibilities. For example, in the racemization of [G QO j]3- a ring-opening path is virtually demanded since all oxalate oxygen atoms exchange with solvent water at a rate faster than that for oxalate exchange but almost equal to that of racemization. 

There has been considerable activity over the past few years in the area of oxygen exchange between chromium(m)-oxalate complexes and water. Now a consistent pattern appears to be emerging, as is shown by the discussion of recent work on oxygpn exchange with the [Cr(ox)(OH2)4]+ cation. In this, as in other chromium(m)- and platinum(n)-oxalate complexes, all the oxalate... 

Ab-initio SCF calculations on the water molecule in various model complexes such as (H20)2, Li+H20, and (Li+)2 H20 show a depletion of the density of the lone pair in the internuclear region for long bonds, while for short bonds, such as Li+ O < a 1.6 A and O(H) O < 2.6-2.7 A, the effect is found to be reversed (Hermansson 1985), in accordance with the observations on oxalic acid dihydrate. The induced polarization of the acceptor density towards the Li+ or H atom is apparently still present for the longer distances, but very diffuse and below the lowest contour of most maps (D. Feil, private communication). Exchange repulsion opposing the attractive effect becomes important for larger ions such as K+, for which the oxygen lone pair penetrates the ion s electron... 

There is brief mention of some iodide-induced exchange of bromate oxygen If correct, this would imply that the rate is limited by (13) or (16). Since, in the reaction with sulphite"the sulphate formed takes oxygen from bromate, that reaction is related to (14), not to (11). In addition to catalysis by chromate , vanadium(V) has been shown to be effective . When oxalate is also present, radicals appear to be produced , as shown by polymerization of acrylonitrile. Vanadium(VI) is suggested as an intermediate . This is of some interest since most authors have regarded one-electron oxidation steps in this whole class of reactions as highly improbable. 

---