Aquation of (N,Ndimethylformamide) pentaamminecobalt (III) perchlorate in aqueous perchlorate media

The first-order rate constants for aquation of Co(NH₃)₅(DMF)³⁺ were determined in aqueous perchlorate media. The rate constants were independent of hydrogen ion concentration and ionic strength, but decreased with increasing perchlorate ion concentration. Proton magnetic resonance studies showed that dimethylformamide was not hydrolyzed to formic acid and dimethylamine in the aquation step. Mass-spectrometer studies showed that cobalt–oxygen bond breaking occurred in 98 percent, or more, of the aquation acts. Enthalpies and entropies of activation were determined. It was concluded that aquation occurred by an I_d mechanism.     

The autoreduction of pertechnetate in aqueous,alkaline solutions

Summary The autoreduction of pertechnetate (⁹⁹TcO₄^-) to Tc(IV/V) alkoxide complexes in aqueous, alkaline, solutions is described. Solutions of sodium pertechnetate (0.01M) reacted with nitrogen and oxygen donor ligands (1.0M) in 2M sodium hydroxide. Solutions containing nitrogen donor ligands (e.g., EDTA) showed the initial formation of lightly colored complexes followed by rapid decomposition in air. In contrast, stable, reduced complexes were formed within minutes of mixing pertechnetate with mono- and disaccharides in strong base, as indicated by a persistent color change. Chemical yields of these reactions were determined by thin layer chromatography or paper chromatography and radiochemically assayed with a Bioscan imaging scanner. Analysis by UV-vis spectroscopy suggested that Tc(IV) or Tc(V) complexes were produced, with the oxidation state dependent on the reducing ligand. These experiments may help explain the reduction of pertechnetate to the soluble complexes that have been found in the Hanford nuclear waste tanks.     

Molecular and electronic structures of bis(dipicolylamine)copper(II) perchlorate and bis[2-(2-pyridylethyl)picolylamine]copper(II) perchlorate

The geometric isomers of bis(dipicolylamine)copper(II) perchlorate, [Cu(dipica)₂](ClO₄)₂, and bis[2-(2-pyridylethyl)picolylamine]copper(II) perchlorate, [Cu(pepica)₂](ClO₄)₂, have been prepared and their molecular structures determined by X-ray diffraction methods. The copper atom of cis-fac-[Cu(dipica)₂](ClO₄)₂, is six coordinate with an amine nitrogen and a pyridyl group of each facial dipica ligand forming a cis coordination plane, and the remaining pyridyl nuclei on the axial sites completing a distorted octahedral structure. The mixed trans-fac- & square-pyramidal-[Cu(dipica)₂](ClO₄)₂ comprises discrete hexacoordinate and pentacoordinate cations. The distorted trans-facial octahedral cation has two picolyl chelates in the equatorial plane and two slightly longer axial pyridyl groups. In the square-pyramidal cation, the basal plane is formed by a meridional tridentate dipicia and a pyridyl of another bidentate dipica ligand, of which the amine group is bound at the apex. The copper ion of trans-fac-[Cu(pepica)₂](ClO₄)₂ is bound by two picolyl chelates in the equatorial plane and two elongated axial pyridyl groups. The electronic structures of these complexes are deduced based on their electronic and e.p.r. spectra. The bonding properties and the formation of the geometric isomers are elucidated.     

Transition-state variation in the nucleophilic substitution reactions of aryl bis(4-methoxyphenyl) phosphates with pyridines in acetonitrile

The kinetics and mechanism of the reactions of Z-aryl bis(4-methoxyphenyl) phosphates, (4-MeOC(6)H(4)O)(2)P(=O)OC(6)H(4)Z, with pyridines (XC(5)H(4)N) are investigated in acetonitrile at 55.0 degrees C. In the case of more basic phenolate leaving groups (Z = 4-Cl, 3-CN), the magnitudes of beta(X) (beta(nuc)) and beta(Z) (beta(lg)) indicate that mechanism changes from a concerted process (beta(X) = 0.22-0.36, beta(Z) = -0.42 to -0.56) for the weakly basic pyridines (X = 3-Cl, 4-CN) to a stepwise process with rate-limiting formation of a trigonal bipyramidal pentacoordinate (TBP-5C) intermediate (beta(X) = 0.09-0.14, beta(Z) = -0.08 to -0.28) for the more basic pyridines (X = 4-NH(2), 3-CH(3)). This proposal is supported by a large negative cross-interaction constant (rho(XZ) = -1.98) for the former and a positive rho(XZ) (+0.97) for the latter processes. In the case of less basic phenolate leaving groups (Z = 3-CN, 4-NO(2)), the unusually small magnitude of beta(Z) values is indicative of a direct backside attack TBP-5C TS in which the two apical sites are occupied by the nucleophile and leaving group, ap(NX)-ap(LZ). The instability of the putative TBP-5C intermediate leading to a concerted displacement is considered to result from relatively strong proximate charge transfer interactions between the pi-lone pairs on the directly bonded equatorial oxygen atoms and the apical bond (n(O)(eq) - sigma(ap)). These are supported by the results of natural bond orbital (NBO) analyses at the NBO-HF/6-311+G//B3LYP/6-311+G level of theory.