REACTION OF AQUACHROMIUM(II) IONS WITH 2,2'-BIPYRIMIDINE - A PULSE-RADIOLYSIS STUDY

Chromium(Ir) ion was generated from Cr(NH3)(6)(3+) and hydrated electr ons in the presence of 2,2'-bipyrimidine (bpm). The reaction of Cr(NH3 )(6)(3+) with e(-)(aq) has k = 4.9 x 10(10) dm(3) mol(-1) s(-1) at 25 degrees C. The Cr(NH3)(6)(2+) so produced loses all the ammonia ligand s with k > 10(5) s(-1). A product complex with an absorption maximum a t around 350 nm was formed in the reaction of chromium(II) ion and bpm . In the presence of a moderate excess of bpm an equilibrium is establ ished on the 0.1 ms timescale with K-app = k(f) [bpm]/k(b). Both the f orward (k(f)) and back (k(b)) reactions are pH dependent in the range 3.5 to 8.6. At pH 3.5 and 8.6, k,has the values of 1.6 x 10(8) and 4.4 x 10(7) dm(3) mol(-1) s(-1) and k(b) of 4.3 x 10(4) and 3.2 x 10(3) s (-1), respectively. The UV spectra of the reaction product(s) indicate that the absorbing product contains chromium(III) and bipyrimidine ra dical ligand. The first acid dissociation constant of chromium(II) ion was determined as 1.4 x 10(-5) mol dm(-3). At pH less than or equal t o 3.5, where all chromium species are predominantly in the aqua form, the proposed reaction scheme is (H2O)(6)Cr2+ + bpm (k-1)reversible arr ow(k1) (H2O)(5)Cr-II(bpm)(2+) (k-2)reversible arrow(k2) (H2O)(4)Cr-II( bpm)(2+) (k-3)reversible arrow(k3) (H2O)(4)Cr-III(bpm(.-))(2+) where [ (H2O)(5)Cr-II(bpm)](2+) and [(H2O)(4)Cr-II(bpm)](2+) represent steady- state intermediates present in stoichiometrically negligible concentra tions. On the 0.1 ms timescale, [(H2O)(4)Cr-III(bpm(.-))](2+) is stabl e but undergoes changes at longer times. The protonated bpm molecule, H(+)bpm, is reduced by 1-hydroxy-1-methylethyl radicals, k = 5 x 10(9) dm(3) mol(-1) s(-1), yielding a diprotonated bpm radical, H(2)bpm(.+) . No reaction was observed between Cr(H2O)(6)(3+) and H(2)bpm(.+) in p ulse radiolysis experiments, which places the rate constant for this r eaction at <2 x 10(6) dm3 mol(-1) s(-1).