EQUILIBRIA IN ACETONE MEDIUM - BINARY-SYSTEMS INVOLVING COBALT(II) HALIDES AND PYRAZOLE-DERIVED LIGANDS

Abstract Stability data associated with CoX 2 -DMPPz binary systems (X = Cl − , Br − , I − ; DMPPz = 3,5-dimethyl-1-phenyl pyrazole) in anhydrous acetone, at 25.00 ± 0.05 °C, have been previously reported [1]. The present communication deals with equilibrium investigations - in the above mentioned solvent and temperature - on CoX 2 -L binary systems, in which L = Pyrazole (Pz) and 3,5-dimethyl pyrazole (DMPz). It is clearly apparent from conductimetric and spectrophotometric measurements that the equlibria involved in these systems can be represented (without taking into account the solvation on the participant species), generically, by the equation: CoX 2 + nL ⇄ CoX 2 L n , and the overall stability constants defined by β n - [CoX 2 L 4 /[CoX 2 ][L] n / The stability constants of the complex species were determined by combining the spectrophotometric method of corresponding solutions with the Fronaeus' computation technique [2]. The evaluated overall, as well as step (K n ) stability constants are ( system , log β n ± limit of error [2], log K n ): CoCl 2 Pz , logβ 1 = 3.69 ± 0.04, logK 1 = 3.69; logβ 2 = 7.07 ± 0.08; logK 2 = 3.38; logβ 3 = 10.24 ± 0.12, logK 3 = 3.17; logβ 4 = 13.54 ± 0.16, log K 4 = 3.30; CoBr 2 Pz , logβ 1 = 4.54 ± 0.05, logK 1 = 4.54; logβ 2 = 8.76 ± 0.10, logK 2 = 4.22; logβ 3 = 12.85 ± 0.15, logK 3 = 4.09; logβ 4 = 16.90 ± 0.20, logK 4 = 4.05; CoI 2 Pz : logβ 1 = 4.00 ± 0.04, logK 1 = 4.00; logβ 2 = 7.69 ± 0.09, logK 2 = 3.69; logβ 3 = 11.22 ± 0.13, logK 3 = 3.53; logβ 4 = 14.79 ± 0.18, logK 4 = 3.57; CoCl 2 DMPz : logβ 1 = 4.53 ± 0.05, logK 1 = 4.53; logβ 2 = 8.78 ± 0.10, logK 2 = 4.25; logβ 3 = 12.69 ± 0.15, logK 3 = 3.91; logβ 4 = 17.02 ± 0.20, logK 4 = 4.33; CoBr 2 DMPz : logβ 1 = 5.22 ± 0.06, logK 1 = 5.22; logβ 2 = 10.13 ± 0.12, logK 2 = 4391; logβ 3 = 14.83 ± 0.17. logK 3 = 4.70; logβ 4 = 19.71 ± 0.23, logK 4 = 4.88; CoI 2 DMPz : logβ 1 = 4.95 ± 0.06, logK 1 = 4.95; logβ 2 = 9.58 ± 0.11, logK 2 = 4.63; logβ 3 = 14.04 ± 0.16, logK 3 = 4.46; logβ 4 = 18.64 ± 0.22, logK 4 = 4.60. A very good compatibility between the calculated formation constants and the pertinent experimental data was achieved for all the studied systems. The listed stability constants, in connection with previously determined values [1] indicate that in acetone medium and with any of the cobalt(II) halides as reference acceptor, the following basicity order holds: DMPz>Pz>DMPPz. Additional evidence in support of this sequences was also obtained from conductivity measurements. On the other hand, for each one of the ligands under consideration, the experimentally observed overall stability trend is: CoBr 2 L n > CoI 2 L n > CoCl 2 L n . it is perhaps worth noting that taking into account only the strength of the Lewis acids the expected stability order would be: CoI 2 L n > CoBr 2 L n > CoCl 2 L n . However, looking exclusively at the ionic radii of the halides the reverse order would be observed, owing to steric hindrance. The higher overall stabilities associated with the CoBr 2 L n system could be ascribed to a best compromise between the two aforementioned opposite effects. For the most part of these systems unusual stability patterns are found(e.g., K 4 > K 3 ) suggesting possible changes in the mode of formation of the complex species as the number of coordinated ligands in increased [1,3]. The financial assistance awarded by CNPq (proc. 305632/78) and FAPESP (Proc.78/0038) is gratefully acknowledged.