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Kinetics of the reaction of arylethanediylidene-bis-dithiocarbazonoate Ni complexes with morpholine in benzene: Substituent and temperature effects



[1-Arylethanediylidene-bis(methyl dithiocarbazonoate) NN′SS′ (−2)]Ni(II) complexes were prepared by condensation of arylglyoxals with methyl dithiocarbazoate to give the corresponding hydrazonoates. The chelation of nickel(II) with these hydrazonoates gave square planar Ni complexes. The k3 values for the substitution of [BOND]SCH3 close to the aryl group with morpholine (Mo) in benzene were found to increase steadily (20–35°C) followed by a sudden drop after 35°C. A continuous decrease in k3 values was observed by further elevation in the temperature. The Arrhenius plot showed a convex curve at the whole temperatures 20–55°C, and negative ΔH# values for the reactions were obtained at 40–55°C. The Hammett plots at the temperature ranges 20–35° and 40–55°C exhibited good straight lines with ρ values of 1.44–0.73 and 1.18–1.25, respectively. The proposed mechanism is a nucleophilic aromatic substitution-like, in which the rate-determining step is the proton transfer process in the temperature range 20–35°C whereas the mechanism in the range 40–55°C passes through the attack of Mo on the carbon carrying the SCH3 group followed by the addition of the second Mo molecule on Ni to form an intermediate, which undergoes elimination of Mo and CH3SH to give a monosubstitution complex. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 27–40, 2012