Kinetics and mechanism of the reaction of hydroxopentaaquarhodium(III) ion with nicotinic acid in aqueous media

Regional Research Laboratory, Bhubaneswar-751 013, India Department of Chemistry, Utkal University, Bhubaneswar-751 004, India Manuscript received 18 December 2003, revised 26 March 2003, accepted 11 June 2004 The kinetics of the reaction of nicotinic acid (H2L+) with hydroxopentaaquarhodium(lll) has been studied over the range 2.58 ≤ 103[H2L+] ≤ 12.9, 3.50 ≤ pH ≤ 4.30, 45º ≤ t ≤ 65º at l = 0.1 mol dm-3 (NaCIO4). The reaction takes place via an outer sphere association between RhOH2+ and HL (conjugate base of H2L+) followed by transformation of the outer into an inner sphere complex (chelate is formed) by slow interchange. The anation rate constant (kan) of RhOH2+ at 25" and I= 0.1 mol drn-3 (NaCIO4) is estimated to he 3.78 x 10-5 s-1. ΔH# and ΔS# for kan path are found to be 33.3 ± 4 kJ moi-1 and -218 12 JK-1 mol-1 respectively. Anation reaction of RhOH2+ follows an Ia path