Structural and Kinetic Studies on the Formation of Platinum(II) and Palladium(II) Complexes with L-Cysteine-Derived Ligands.

Complex-formation reactions of the L-cysteine-derived ligands (alpha)N-acetyl-S-methylene-(2'-pyridine)-L-cysteine (py-CH(2)-accys) and (alpha)N-acetyl-S-ethylene-2-(2'-pyridine)-L-cysteine (py-C(2)H(4)-accys) with [Pt(en)(H(2)O)(2)](2+) and [Pd(en)(H(2)O)(2)](2+) were investigated structurally by NMR spectroscopy and kinetically by UV-vis and stopped-flow spectrophotometry. The complexes [Pt(en)(py-CH(2)-accys-S,N)](NO(3))(2) (1) and [Pt(en)(py-C(2)H(4)-accys-S,N)](NO(3))(2) (2) were isolated and purified by reversed-phase HPLC. NMR spectroscopy revealed an S-thioether, N-pyridyl chelation mode with five- and six-membered chelate rings for 1 and 2 and their Pd(II) analogues 3 and 4.

Structural, Spectroscopic, Thermodynamic and Kinetic Properties of Copper(II) Complexes with Tripodal Tetraamines.

Spectroscopic, thermodynamic, and kinetic measurements have been made on aqueous solutions of copper(II) complexes of hexamethylated tren and trimethylated tren (one methylation per primary amine group of tren) with the objective of correlating the influence of geometry (trigonal bipyramidal, evident from UV/vis spectroscopy) and N-alkyl substitution in the ligand on these inherent properties. At 25.0 degrees C the protonation constants of Me(3)tren are not significantly different from those of tren and Me(6)tren, and the stability constant for the Cu(II) complex is of the same order of magnitude as that for the [Cu(tren)(H(2)O)](2+) complex ion.

High-Pressure Oxygen-17 NMR Study of the Dihydroxo-Bridged Rhodium(III) Hydrolytic Dimer. Mechanistic Evidence for Limiting Dissociative Water Exchange Pathways.

Rate constants and activation parameters for water exchange on the Rh(III) hydrolytic dimer [(H(2)O)(4)Rh(&mgr;-OH)(2)Rh(H(2)O)(4)](4+) were determined using (17)O NMR spectroscopy as a function of temperature (308-323 K) and pressure (0.1-150 MPa). This represents the first variable-pressure (17)O NMR study conducted on a polynuclear metal aqua ion.

Fixation of CO(2) by Zinc(II) Chelates in Alcoholic Medium. X-ray Structures of {[Zn(cyclen)](3)(&mgr;(3)-CO(3))}(ClO(4))(4) and [Zn(cyclen)EtOH](ClO(4))(2).

The fixation of CO(2) on Zn(II) chelates in alcoholic medium was studied in efforts to synthesize alkyl carbonates. In the presence of a base, such as NEt(3), the tetraazacyclododecane complex of Zn(II) in alcoholic solution binds CO(2) and forms a &mgr;(3)-carbonate complex. This complex was characterized by NMR spectroscopy and X-ray analysis.

Evidence for the Unexpected Associative Displacement of Adenosyl by Cyanide in Coenzyme B(12).

The reaction of coenzyme B(12) (adenosylcobalamin) with cyanide has been reinvestigated in detail using spectroscopic and kinetic techniques. It has been shown that this reaction proceeds in one kinetically observable step, contradicting previous findings, with rate-determining attack of the first cyanide (k = (7.4 +/- 0.

Complex Formation and Ligand Substitution Reactions of (2-Picolylamine)palladium(II) with Various Biologically Relevant Ligands. Characterization of (2-Picolylamine)(1,1-cyclobutanedicarboxylato)palladium(II).

The influence of a 2-picolylamine (pic) ligand on the reactivity of Pd(II) complexes was investigated by detailed equilibrium and kinetic studies. Reactions of [Pd(pic)(H(2)O)(2)](2+) with chloride, 1,1-cyclobutanedicarboxylic acid (CBDCAH(2)), inosine (ino), and inosine 5'-monophosphate (5'-IMP) were studied. A significantly higher reactivity for the first reaction step involving the displacement of one coordinated solvent molecule on [Pd(pic)(H(2)O)(2)](2+) was observed for the nucleoside inosine (k(10)( degrees )(C) = 25 400 +/- 200 M(-)(1) s(-)(1)) than for the nucleotide 5'-IMP (k(10)( degrees )(C) = 7100 +/- 300 M(-)(1) s(-)(1)) and for CBDCAH(-) (k(25)( degrees )(C) = 5380 +/- 70 M(-)(1) s(-)(1); DeltaH() = 54 +/- 2 kJ mol(-)(1); DeltaS() = 10 +/- 4 J K(-)(1) mol(-)(1); DeltaV() = -0.

Mechanistic Studies of the Formation and Decay of Diiron(III) Peroxo Complexes in the Reaction of Diiron(II) Precursors with Dioxygen.

Mechanistic studies of the reactions of three analogous alkoxo-bridged diiron(II) complexes with O(2) have been carried out. The compounds, which differ primarily in the steric accessibility of dioxygen to the diiron(II) center, form metastable &mgr;-peroxo intermediates when studied at low temperature. At ambient temperatures, these intermediates decay to form (&mgr;-oxo)polyiron(III) products.

Comparative Kinetic Analysis of Reversible Intermolecular Electron-Transfer Reactions between a Series of Pentaammineruthenium Complexes and Cytochrome c.

In this kinetic and thermodynamic study, the reversible outer-sphere electron-transfer reactions between a series of Ru(NH(3))(5)L(3+/2+) complexes (L = etpy, py, lut) (etpy = 4-ethylpyridine; py = pyridine; lut = 3,5-lutidine) and cytochrome c were investigated as a function of ionic strength, buffer, pH, temperature, and pressure. Due to the low driving forces of these systems, it was possible to study all the reactions in both redox directions. The observed rate constants for various L are correlated on the basis of the ability of ligands on the ruthenium complex to penetrate the heme groove on cytochrome c.