Ligand substitution dynamics (1973)
AuthorsCouldwell, Margaret Claireshow all
Tetraaminecobalt(III) and chromium(III) complexes of some linear polyamine ligands have been studied, primarily from a reaction rate point of view. The complexes trans-[CrCl₂(tmd)₂]Br, trans-[CrBr₂(tmd)₂]X (X = Br, ClO₄), trans-[CrCl₂(en) (tmd)]ClO₄, trans- [CoCl₂ (en) (tmd) ]X (X = Cl.H₂O, ClO₄) and trans- [CoCl₂(NH₃) (dien) ]ClO₄ have been prepared to investigate their kinetics of acid hydrolysis in aqueous HNO₃ From these, the cations trans-CrX₂ (tmd)₂⁺, (X = Cl,Br), trans-MCl₂(en)(tmd)⁺ (M = Cr, Co), trans-CoC1₂(NH₃)(dien)⁺, trans-CrX (tmd)₂(OH₂)²⁺ (X = Cl, Br), trans-CrCl(en)(tmd)(OH₂)²⁺, cis-CoCl(en)(tmd)(OH₂)²⁺, trans-Cr(AA)(tmd)(OH₂)₂³⁺ (AA = en, tmd), trans-Co(NH₃)(dien)(OH₂)²⁺,trans-Cr(AA)(tmd)(OH₂)₂³⁺ (AA = en, tmd), cis-Co(en)(tmd)(OH₂)₂³⁺cis-Co(NH₃)(dien)-(OH₂)₂³⁺ Cr(tmd)(OH₂)₄³⁺trans-Co(OH)₂(en)(tmd)⁺ and trans-Co(OH)₂(NH₃) (dien)⁺ have been isolated and characterised in solution. The complexes trans-CrX₂(tmd)₂⁺ (X = Cl, Br) and trans-CrC1₂(en)(tmd)⁺ aquate with complete retention of configuration to their corresponding trans-aquachloro isomer. At 298.2 K the first-order rate constants are 10⁵k (sec-¹) = 2.08±0.06 (0.1 F HNO₃), 36.2 ±1.3 (0.3 F HN0₃), 1.93±0.09 (0.4, F HN0₃), respectively. The secondary hydrolysis of these cations is complicated by isomerisation and concurrent Cr-N bond rupture. The first-order halide release rate constants for trans-CrX(tmd)₂(OH₂)²⁺ (X = Cl, Br) and trans-CrCl(en)(tmd)(OH₂)²⁺ in 1.0 F HNO₃ at 318.2 K are 10⁵k(sec-¹) = 2.88±0.06, 13.6±0.8, 3.15±0.18, respectively. In the primary hydrolysis step the cation CoCl₂(en)(tmd)⁺, 66±5% cis-CoCl(en)(tmd)(OH₂)²⁺ is produced, and the first-order rate constant for this in 0.3 F HNO₃ at 298.2 K is 10⁵k (sec-¹) = 39.6±1.2. The first-order halide release rate constant for cis-CoCl(en)(tmd)(OH₂)²⁺ in 1.0 F HNO₃ at 298.2 K is 10⁵k(sec ) = 10.3±0.2. The rate of acid hydrolysis of has been measured both spectrophotometrically and by chloride release. In 0.3 F HNO₃ at 298.2 K, the first-order rate constants for the primary hydrolysis are 10⁵k spectro(sec-¹) = 39.8±0.3 and 10⁵kcl (sec-¹) = 52.1±0.8. These data have been interpreted in terms of a mechanism whereby the trans-dichloro cation hydrolyses in acid solution to produce three aquachloro species. The major component has assigned fac-a,b,edf,c-CoCl(NH₃)(dien)(OH₂)²⁺ configuration, and the rate of hydrolysis of this isomer has been measured by chloride release, and in 1.0 F HNO₃ at 298.2 K, 10⁵kcl (sec-¹) = 7.89±0.28. For each of these kinetic studies the activation parameters have been calculated and are discussed. The acid dissociation constants for the equilibria trans-Cr-(AA) (BB) (OH₂)₂³⁺(ag)+H₂O ⇌ trans-Cr(OH) (AA) (BB) (OH₂)²⁺ (ag) + H₃O (ag) pK₁ trans-Cr(OH) (AA) (BB) (OH₂)²⁺ (ag) + H₂O ⇌ trans-Cr(OH)₂ (AA) (BB) ⁺ (ag) + H₃O (ag) pK₂ where AA = BB = en; AA = en, BB tmd; AA = BB = trod have been determined. Similar ts were obtained each complex: pK₁ ca. 4.2, pK₂ ca. 7.6; 1 F NaNO₃, 298.2 K. The results are compared with pK data for related complexes of chromium and cobalt. The complex Cr(OH)(en)₂(OH₂)²⁺ undergoes extensive Cr-N bond rupture to yield deaminated products. The crystal structure of one isomer of [Co(ox) (NH₃)(dien)]-NO₃ has been determined. The three nitrogen atoms of the diethylenetriamine ligand are in a plane with one oxygen atom of the oxalate ligand, while the second oxalate oxygen atom is trans to the ammonia group. The orientation about the secondary nitrogen group of the diethylenetriamine ligand is such that the NH proton is adjacent to the coordinated ammonia ligand.