Werner Marty

Early stages of the hydrolysis of chromium(III) in aqueous solution—VI. Kinetics of intramolecular interconversion between singly- and doubly-bridged hydrolytic dimers☆

Abstract The singly-bridged hydrolytic dimer [(H2O)5Cr(μ-OH)2)Cr(OH2)5]5+ (SBD), undergoes a ring closure reaction which transforms it into the doubly-bridged dimer, [(H2O)4Cr(μ-OH)2Cr(OH2)4]4+ (DBD). The kinetics of this conversion have been studied over the pH range 0–4.93, temperature range 288.15–313.35 K and I = 1.0 M (adjusted with either LiClO4 or NaClO4). The pH dependence of the first-order rate constant, kobs, was interpreted in terms of three distinct pathways involving ring closure within fully protonated, mono- and doubly-deprotonated SBD and two pathways accounting for acid-catalysed and uncatalysed cleavage of DBD which are important at low pH. Least-squares analysis of the data permitted the determination of rate constants for each of these pathways and the first two acid dissociation constants of the SBD. A reactivity increase of ca four-fold observed on monodeprotonation of SBD was suggested to reflect, at least in part, the greater nucleophilicity of coordinated OH−, cf. coordinated H2O. The more substantial rate enhancement of 25-fold resulting from double deprotonation of SBD was concluded to be due to an increase in the lability of the chromium(III) centre at which substitution occurs. The activation parameters for all pathways are consistent with an associative substitution mechanism, either A or Ia.

Synthesis and crystal structures of complexes of MnII, CuII and ZnII with the new bis(tridentate) chelate 2,3,5,6-tetrakis(aminomethyl)pyrazine

The new bis(tridentate) chelate 2,3,5,6-tetrakis(aminomethyl)pyrazine (tampyz) has been prepared. It forms a quasi-linear one-dimensional polymeric complex with MnCl2 in water. [{Mn(tampyz)Cl2·2H2O}∞]. The crystal structure reveals that it is the first infinite co-ordination polymer with a regular metal–ligand arrangement where the metal atom is fully co-ordinated by all six functional groups of the ligand. The binuclear complex [Cu2(tampyz)Cl2]Cl2·2.25H2O was obtained by the reaction of Cu(O3SC6H4Me-P)2·5H2O and [H4tampyz]Cl4 in water, and its crystal structure determined. The copper atoms can be considered to be either four-co-ordinate in a square-planar environment, or six-co-ordinate with two weak apical Cu ⋯ Cl bonds so forming a ladder-type polymer. The binuclear complex [Cl2Zn(tampyz)ZnCl2] was prepared by the reaction of ZnCl2 and tampyz in water. Its crystal structure reveals that the zinc atoms are five-co-ordinate in a trigonal-bipyramidal arrangement. The two Zn–Cl bonds are equivalent, as expected for a d10 complex.

Anionopentaaminecobalt(III) complexes with polyamine ligands: Part 30. The base hydrolysis kinetics and structure determination of some [CoCl(N)5]2+ complexes containing pyridine ligands

The base hydrolysis rates ( k OH , M −1 s −1 , I =0.1 M, 25°C) have been measured for cis- [CoBr(en) 2 (py)] 2+ ( k OH =3.33×10 3 ), cis- [CoBr(en) 2 (d 5 py)] 2+ (3.34×10 3 ), cis- [CoCl(en) 2 (4-NH 2 -py)] 2+ (78.2), cis- [CoCl(en) 2 (4-N(Me) 2 -py)] 2+ (77.0), cis- [CoCl(en) 2 (4-CNpy)] 2+ (326), trans- [CoCl(tn) 2 (py)] 2+ (3.87×10 3 ), pf -[CoCl(dien)(NH 3 ) 2 ] 2+ (252) and pf- [CoCl(dien)(py) 2 ] 2+ (2.77×10 4 ) using pH-stat techniques. The crystal structures of pf- [CoCl(dien)(py) 2 ](NO 3 )(ClO 4 ) and p 3 f- [CoCl(2,3-tri)(py) 2 ]ZnCl 4 have been determined. The k OH values for py complexes are significantly greater than those observed for the analogous methylamine complexes and the rate enhancement is attributed to weak π[3d(Co)]→π*[pyridine] back donation.