Preparation, Characterization, and Aquation Kinetics of Pyridine N ‐Oxide Complexes of Chromium(III)

A series of (H 2 O) 5 Cr(X‐pyO) 3+ ions (pyO = pyridine N ‐oxide, X = H, 3‐CH 3 , 4‐CH 3 , 4‐OCH 3 , 4‐NO 2 ) were prepared by the reduction of the corresponding pyridine N ‐oxide adducts of diperoxochromium(VI) species with acidic ferrous perchlorate. The (H 2 O) 5 Cr(X‐pyO) 3+ complexes undergo aquation to yield Cr(H 2 O) 6 3+ and X‐pyO according to the rate law k obs = k o  +  k –1 [H + ] –1 . The values of the rate constants extrapolated to 298 K at 1.0 M ionic strength are: k 0 = 2.80 × 10 –6  s –1 , k –1 = 1.86 × 10 –8 M  s –1 (X = 4‐NO 2 ); 7.80 × 10 –8 , 6.27 × 10 –10 (H);4.80 × 10 –8 , 3.20 × 10 –10 (3‐CH 3 ); 3.05 × 10 –8 , 1.60 × 10 –10 (4‐CH 3 ); and 2.37 × 10 –9 , 4.76 × 10 –11 (4‐OCH 3 ). The reaction of the 4‐OCH 3 complex exhibits two additional terms in the rate law, k 1 [H + ] +  k –2 [H + ] –2 . The binding of 4‐OCH 3 –pyO to chromium is suggested to take place through the methoxy group. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)