Mechanisms of the titanium(III) reduction of tris(1,10-phenanthroline)cobalt(III), tris(oxalato)cobaltate(III), and tris(1,10-phenanthroline)iron(III) in aqueous solutions

Ige

et al. 2006

Self Cite

The kinetics of the reactions between Fe(phen) 3 2+ [phen = tris-(1,10) phenanthroline] and Co(CN) 5 X 3À (X = Cl, Br or I) have been investigated in aqueous acidic solutions at I = 0.1 mol dm )3 (NaCl/HCl). The reactions were carried out at a fixed acid concentration ([H + ] = 0.01 mol dm )3 ) and the second-order rate constants for the reactions at 25°C were within the range of (0.151-1.117) dm 3 mol )1 s )1 . Ion-pair constants K ip for these reactions, taking into consideration the protonation of the cobalt complexes, were 5.19 Â 10 4 , 3.00 Â 10 2 and 4.02 Â 10 4 mol )1 dm )3 for X = Cl, Br and I, respectively. Activation parameters measured for these systems were as follows: DH* (kJ K )1 mol )1 ) = 94.3 ± 0.6, 97.3 ± 1.0 and 109.1 ± 0.4; DS* (J K )1 ) = 69.1 ± 1.9, 74.9 ± 3.2 and 112.3 ± 1.3; DG* (kJ) = 73.7 ± 0.6, 75.0 ± 1.0 and 75.7 ± 0.4; E a (kJ) = 96.9 ± 0.3, 99.8 ± 0.4, and 122.9 ± 0.3; A (dm 3 mol )1 s )1 ) = (7.079 ± 0.035) Â 10 16 , (1.413 ± 0.011) Â 10 17 , and (9.772 ± 0.027) Â 10 20 for X = Cl, Br, and I respectively. An outer -sphere mechanism is proposed for all the reactions.

Section: Methodssupporting

confidence: 82%

“…They are very stable and inert, low-spin t 2 g 6 complexes [1][2][3]. Reactions of Co(CN) 5 X 3À (X = Cl, Br or I) with Ti(III) [4], V(II) [5] and Ru(II) (O. T. Ayinde, unpublished work) have been investigated and concluded to proceed by an outer-sphere mechanism since k sub < <k e.t .…”

Section: Introductionmentioning

confidence: 99%

Ion-pair formation in the outer-sphere electron transfer reactions between tris-(1, 10) phenanthroline iron(II) and the halopentacyanocobaltate(III) complexes in aqueous acidic solutions

Ige

et al. 2006

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“…Nonnegligible k 0 contributions were also observed in all the reductions of these oxidants by Ti III . Similar observations have been reported in the literature by earlier workers [1,[19][20].…”

Section: Resultssupporting

confidence: 93%

“…K a values ‡ 1.0 Â 10 )2 mol dm )3 have been obtained by some workers [19,21], and such high values have been used to suggest precursor complex formation and hence an inner-sphere mechanism [22]. The ammonia ligands in [Co(NH 3 ) 5 NO 2 ] 2+ are more weakly bound to cobalt than the cyano ligands in [Co(CN) 5 NO 2 ] 3) [23,24] and therefore greater reactivity is expected from [Co(NH 3 ) 5 NO 2 ] 2+ .…”

Section: Resultsmentioning

confidence: 76%

Outer-Sphere electron-transfer and the effect of linkage isomerism in the titanium(III) reduction of nitro-pentacyanocobaltate(III) and nitro-, and nitrito-pentaamminecobalt(III) complexes in aqueous acidic media

Adegoke

2007

The reductions of [Co(CN) 5 NO 2 ] 3) , [Co(NH 3 ) 5 NO 2 ] 2+ and [Co(NH 3 ) 5 ONO] 2+ , by Ti III in aqueous acidic solution have been studied spectrophotometrically. Kinetic studies were carried out using conventional techniques at an ionic strength of 1.0 mol dm )3 (LiCl/HCl) at 25.0 ± 0.1°C and acid concentrations between 0.015 and 0.100 mol dm )3 . The second-order rate constant is inverse-acid dependent and is described by the limiting rate law:-where k=k¢K a and K a is the hydrolytic equilibrium constant for [Ti(H 2 O) 6 ] 3+ . Values of k 0 obtained for [Co(CN) 5 NO 2 ] 3) , [Co(NH 3 ) 5 NO 2 ] 2+ and [Co(NH 3 ) 5 ONO] 2+ are (1.31 ± 0.05) Â 10 )2 dm 3 mol )1 s )1 , (4.53 ± 0.08) Â 10 )2 dm 3 mol )1 s )1 and (1.7 ± 0.08) Â 10 )2 dm 3 mol )1 s )1 respectively, while the corresponding k¢ values from reductions by TiOH 2+ are 10.27 ± 0.45 dm 3 mol )1 s )1 , 14.99 ± 0.70 dm 3 mol )1 s )1 and 17.93 ± 0.78 dm 3 mol )1 s )1 respectively. Values of K a obtained for the three complexes lie in the range (1-2) Â 10 )3 mol dm )3 which suggest an outer-sphere mechanism.

“…Each complex was purified by dissolving in water and recrystallising from cold EtOH. The complexes were characterized as follows: 5 Cl ¼k max 393 nm; e¼ 160 M À1 cm À1 ; IR band À 2111 cm À1 K 3 ½Co(CN) 5 Br ¼k max 398 nm; e¼ 190 M À1 cm À1 ; IR band À 2114 cm À1 K 3 ½Co(CN) 5 I ¼k max 440 nm; e¼ 200 M À1 cm À1 ; IR band À 2122 cm À1 Fe(Phen) 2þ 3 was synthesised in the form Fe(Phen) 3 (ClO 4 ) 2 and characterized by its visible spectrum e¼ 1:09 Â 10 4 m )1 cm )1 , k max 510 nm [14]. These values are in agreement with the literature [15].…”

Section: Methodsmentioning

confidence: 99%

Protonation of Chloro-, Bromo- and Iodo-pentacyanocobaltate(III) Complexes

Ige

et al. 2006

Self Cite

The reactions between Fe(Phen) 2þ 3 [phen=tris-(1,10) phenanthroline] and Co(CN) 5 X 3) (X=Cl, Br or I) have been studied in aqueous acidic solutions at 25°C and ionic strength in the range I=0.001-0.02 mol dm )3 (NaCl/HCl). Plots of k 2 versus p I, applying Debye-Huckel Theory, gave the values )1.79±0.18, )1.65±0.18 and 1.81±0.10 as the product of charges ðZ A Z B Þ for the reactions of Fe(Phen) 2þ 3 with the chloro-, bromo-and iodo-complexes respectively. Z A Z B of % À2 suggests that the charge on these Co III complexes cannot be )3 but is )1. This suggests the possibility of protonation of these Co III complexes. Protonation was investigated over the range [H + ]=0.0001 )0.06 mol dm )3 and the protonation constants K a obtained are 1.22 · 10 3 , 7.31 · 10 3 and 9.90 · 10 2 dm 6 mol )3 for X=Cl, Br and I, respectively.