STRUCTURE OF THE SHORT-LIVED INTERMEDIATE FORMED DURING THE METAL SUBSTITUTION-REACTION OF THE MERCURY(II) PORPHYRIN COMPLEX WITH COBALT(II) ION IN AQUEOUS-SOLUTION DETERMINED BY THE STOPPED-FLOW EXAFS METHOD

The local structure around the cobalt(II) ion in the reaction intermed iate formed during the metal substitution reaction of the homodinuclea r mercury(II) porphyrin (5,10,15,20-tetrakis(4-sulfonatophenyl)porphyr in; H(2)tpps(4-)) complex with a cobalt(II) ion in an acetate buffer h as been determined by the stopped-flow EXAFS method. The structure of the reactant and the product of the above reaction has also been deter mined by the same method. The coordination geometry around the cobalt( II) ion in the heterodinuclear intermediate, [Hg(tpps)Co-II](2-) is si x-coordinate octahedral with four additional water and/or acetate oxyg en atoms. The Co-II- N and Co-II-O bond lengths in the intermediate ar e 212(2) and 221(1) pm, respectively. The product, [Co-II(tpps)](4-), has a six-coordinate octahedral structure, the Co-II-N and Co-II-O bon d lengths being 203(1) and 215(1) pm, respectively. The Co-II-N bond l ength in the intermediate is ca. 9 pm longer than that in the product. The Co-II-O bond length in the intermediate is also ca. 9 pm longer t han that of 212(1) pm in the reactant, the cobalt(II) acetato complex, and ca. 6 pm longer than that in the product. The longer Co-II-O bond in the intermediate as compared to those in the reactant and in the p roduct appears to be responsible for the instability of the intermedia te. The oxidized product, [Co-III(tpps)](3-), has a six-coordinate str ucture with two additional Co-III-O bonds. The Co-III-N and Co-III-O b ond lengths are 189(1) and 197(2) pm, respectively, and are much short er than those in [Co-II(tpps)](4-).