Albert Haim

KINETICS OF FORMATION AND DISSOCIATION OF COMPLEXES OF PENTACYANOFERRATE(II) WITH BENZONITRILE, DICYANOBENZENES AND CYANOPYRIDINES

Abstract A series of organonitrile complexes of pentacyanoferrate(II) have been prepared in aqueous solution by the reacion of Fe(CN) 5 OH 3− 2 with the appropriate nitrile. The complexes exhibit a metal to ligand charge transfer absorption at 341 (benzonitrile), 413 (1,2-dicyanobenzene), 381 (1,3-dicyanobenzene) and 419 nm (1,4-dicyanobenzene). The rate constants for the formation of these complexes at 25° and ionic strength 0.10 M (lithium perchlorate) are (same order as above); 270, 453, 638 and 410 M −1 sec −1 . The rate constants for the dissociation under the same conditions are: 0.115, 0.0812, 0.0786 and 0.0663 sec −1 . The reactions of Fe(CN) 5 OH 3− 2 with 2-, 3-, and 4- cyanopyridine proceed in two stages. The first corresponds to the formation of a mixture of pyridine-bound and nitrile-bound linkage isomers, and the second to the linkage isomerization of the unstable isomer. For 3- and 4-cyanopyridine, the unstable isomers are nitrile-bond, but for 2-cyanopyridine, because of the steric hindrance between the nitrile group in thte 2-position and the cis cyanide groups attached to the iron, the unstable isomer is pyridine-bound. The metal to ligand charge transfer bands of the nitrile-bound isomers are at 387, 370 and 405 nm for 2-, 3-, and 4-cyanopyridine, respectively. The bands for the pyridine bound isomers are at 470, 414 and 477 nm. The rate constants for the formation and the dissociation of the nitrile bound isomers are: 365-441 M −1 sec −1 and 9.3 × 10 −2 sec −1 (2-cyanopyridine); 223 M −1 sec −1 and 0.117 sec −1 (3-cyanopyridine); 235 M −1 sec −1 and 9.7 × 10 −2 sec −1 (4-cyanopyridine). The rate constants for the pyridine bound isomers are, in the same order: 155-231 M −1 sec −1 and 1.18 sec −1 ; 413 M −1 sec −1 and 2.80 × 10 −3 sec −1 ; 383 M −1 sec −1 and 1.02 × 10 −3 sec −1 . The rate and equilibrium data for the dissociation of these complexes exhibit a linear free energy relationship of slope 0.98, indicating a dissociative mechanism. An intramolecular pathway for linkage isomerization is absent in the 4-cyanopyridine systems and makes, at most, an 18% contribution in the 2-cyanopyridine system.

Intramolecular Electron Transfer Reactions of Ion Pairs: Thermal, Optical, and Photochemical Pathways

Abstract It has been recognized for several years that bimolecular electron transfer reactions1 (as well as other reactions2) proceed via a sequence of elementary steps, namely, formation of the precursor complex, intramolecular electron transfer within the precursor complex, and dissociation of the successor complex. For outer-sphere reactions, the precursor and successor complexes are ion pairs or outer-sphere complexes. For inner-sphere reactions, the precursor and successor complexes are binuclear complexes in which a bridging ligand connects the two metal centers. Under conditions where the stability of the precursor complex is low (Q IP or Qp « 1) and the electron transfer step ket is rate determining, experimental kinetic measurements yield second-order rate constants k exp which are equal to the product of the equilibrium constant for the formation of the precursor complex and the rate constant for electron transfer, k exp=Q IP k et or Qpk et.3 The overwhelming majority of electron transfer reacti...

Synthesis and properties of the binuclear complexes and

The binuclear complexes (NC5FeLM(NH3)5 (L = 4,4′bipyridine, M =Ru, Rh) were prepared by the rapid reaction between M(NH3)5L3+ and Fe(CN)5OH3−2. The compound Na[(NC)5FeL′Ru(NH3)5] (L′ = pyrazine) was prepared by the reaction between Fe(CN)5NH3−3 and ru(NH3)5L2+. Oxidation of Na[(NC)5FeL′Ru(NH3)5] with cerium(IV) or peroxydisulfate yielded (NC)5FeL′Ru(NH3)5. The reaction between Fe(CN)5NH3−3 and Rh(NH3)5L′3+ produced (NC)5FeL′Rh(NH3)5. Cobalt analogs of the FeRu complexes were prepared by reaction of Co(CN)5OH3−2 with Ru(NH3)5L2+ and Ru(NH3)5LL′2+. The compounds were characterized by solid state spectroscopic measurements (KBr pellets) in the visible, near infrared, and infrared regions. The wavelengths of the maxima of the metal to ligand charge transfer bands, cyanide stretching bands and ammonia deformation bands of the title compounds, and comparisons with the corresponding bands of their rhodium and cobalt analogs, are used to assign a valence-trapped formulation with localized iron(II) and ruthenium(III) oxidation states to the title com[ounds

Yields of singlet dioxygen produced by the reaction between the excited state of tris(bipyridine)ruthenium(II) and triplet dioxygen in various solvents

Abstract The yields of singlet dioxygen produced in the reaction between the excited state of tris(bipyridine)ruthenium(II) and triplet dioxygen in water, deuterium oxide, methanol and acetonitrile were measured by a steady state photolysis technique which involves the trapping of singlet dioxygen with 9,10-anthracenedipropionate (aqueous solutions) or 1,3-diphenyl-isobenzofuran (non-aqueous solutions) and following the disappearance of the trap as a function of time. Rate constants for the decay of *Ru(bpy) 3 2+ (bpy, bipyridine) and for its reactions with triplet dioxygen were measured by laser time-resolved techniques. Singlet oxygen is formed with unit efficiency in all the solvents studied.

Syntheses of the linkage isomers (H2O)5CrNCCo(CN)5 and (H2O)5CrCNCo(CN)5

Abstract The slow reaction of Cr(H 2 O) 6 3+ with Co(CN) 6 3− produces (H 2 O)CrNCCo(CN) 5 . The rapid reaction of Co(CN) 5 N 3 3− with nitrious acid in the presence of Cr)OH 2 ) 5 CN 2+ produces (H 2 O) 5 CrCNCo(CN) 5 . The two binuclear complexes were separated and purified by cation and anion exchange procedures. The orientation of the bridging cyanide in the isomeric complexes was determined by a spectroscopic technique. For (H 2 O) 5 CrNCCo(CN) 5 the 4 A 2 g → 4 T 2 g and 4 A 2 g → 4 T 1 g of the chromium moiety are at 560 and 401 nm, respectively, and the 1 A 1 g → 1 T 1 g of the cobalt moiety is at 310 nm. For (H 2 O) 5 CrCNCo(CN) 5 the 4 A 2 g → 4 T 2 g band is at 552 nm, but the 4 A 2 g → 4 T 1 g absorption of the chromium is obscured by the 1 A 1 g → 1 T 1 g transition of the cobalt at 347 nm. The base hydrolysis of (H 2 O) 5 CNCCo(CN) 5 produces Co(CN) 6 3− and “chromite” quantitatively. The vase hydrolysis of (H 2 O) 5 CrCNCo(CN) 5 produces chromite and an unidentified cobalt(III) species that absorbs at 345–347 nm.

Intramolecular electron transfer from RuII(EDTA)2− to ComIII(NH3)53+ via nitrogen heterocycle bridging ligands

Abstract A series of binuclear complexes of formula (EDTA)Ru III LCo III (NH 3 ) 5 2+ (EDTA= ethylenediaminetetraacetate, L=pyrazine, 4,4′-bipyridine, 3,3′-dimethyl-4,4′-bipyridine, trans -l,2-bis(4-pyridyl)ethylene, 1,4-bis(4-pyridyl)butadyne) were prepared in aqueous solution by reaction of Ru III (EDTA)OH 2 - with Co(NH 3 ) 5 L 3+ . The reduction potentials of the binuclear complexes were measured. Reactions of the binuclear complexes with ascorbic acid or dithionite result in the preferential reduction of Ru(III) to Ru(II) and produce (EDTA)Ru II LCo III (NH 3 ) 5 + . The latter undergo intramolecular electron transfer from Ru(II) to Co(III) with rate constants (25.0 °C, I =0.20 M) 22.7±0.4, 0.64±0.02, 0.067±0.002, 0.21±0.01 and 0.039±0.001 s -1 (in the same order as above). The decrease in rate constant with increasing distance between metal centers is ascribed to the increase in solvent reorganization energy with increasing metal to metal distance. The reorganization energies corrected for the solvent contribution (ellipsoidal cavity model) have values of 10.6, 10.5, 11.8, 10.5 and 10.9 kcal mol -1 . Except for 3,3′-dimethyl-4,4′-bipyridine, it is suggested that the electron transfer is adiabatic. The results are compared with results from previous studies with (NC) 5 Fe III LCo III L(NH 3 ) 5 .

KINETICS AND MECHANISM OF THE OXIDATION OF AMMINERUTHENIUM(II) COMPLEXES BY BROMINE

Abstract The reactions of Ru(NH3)5py2+, Ru(NH3)4bpy2+, Ru2(NH3)10pz5+, RuRh(NH3)10pz5+ and Ru(NH3)5pz2+ with bromine are first-order in ruthenium and first-order in bromine. The rates decrease with increasing bromide ion concentration and, except for Ru(NH3)5pz2+, are independent of hydrogen ion concentration. The reactions are postulated to proceed via outer-sphere, one-electron transfer from Ru(II) to Br2 with the formation of Br2− as a reactive intermediate. The bromide inhibition is ascribed to the formation of Br3− which is unreactive in outer-sphere reactions because of the barrier imposed by the need to undergo reductive cleavage. The reaction of Ru(NH3)5pz2+ is inhibited by hydrogen ions. The hydrogen ion dependence shows that Ru(NH3)5pzH3+ has a pKa of 2.49 and is at least 500 times less reactive than Ru(NH3)5pz2+. The reaction of Ru2(NH3)10pz4+ with bromine is biphasic. The second phase has a rate identical to that of the Ru2(NH3)10pz5+-Br2 reaction. A detailed analysis shows that the reaction of Ru2(NH3)10pz4+ with bromine proceeds by a sequence of one-electron steps, Br2− being produced as an intermediate. A linear free energy relationship between rate constants and equilibrium constants, obeyed for all the reactions studied, provides an estimate of ∼ 1.5 × 102 M−1 s−1 for the self-exchange rate constant of the Br2/Br2− couple.

Kinetics and mechanism of reduction of sperm-whale metmyoglobin by dithionite ion☆

Abstract The kinetics of reduction of sperm-whale aquometmyoglobin by excess dithionite ion was studied at 25°C, pH 6.9–9.8, and ionic strength 0.50 M (adjusted with potassium nitrate). The dependence of the pseudo first-order rate constant for the disappearance of metmyoglobin on hydrogen and dithionite ion concentrations is ( k 1 [S 2 O 4 2− ] + k 2 [S 2 O 4 2− ] 1 2 )[H + ]/( K a + [H + ]), where k 1 = 52 ± 11 M −1 s −1 , k 2 = 40.9 ± 2.3 M 1 2 s −1 , and K a (the ionization constant of the water coordinated to the iron) = (5.79 ± 0.49) × 10 −10 M. The rate law is interpreted on the basis of parallel pathways for the reaction of aquometmyoglobin with S 2 O 4 2− and SO 2 − , the hydroxometmyoglobin in equilibrium with the aquo form being unreactive. From a comparison of the rate constants for anation of aquometmyoglobin and the rate constant for reduction by SO 2 − , it is inferred that an outer-sphere redox mechanism is operative. It is postulated that the activation process for reduction of aquometmyoglobin requires considerable stretching of the Fe-OH 2 bond, and this model is utilized to assign an outer-sphere mechanism to the reduction by S 2 O 4 2− . The dithionite reduction of cyanometmyoglobin proceeds in two stages. The first stage proceeds at a rate that is dependent on dithionite concentration and corresponds to the outer-sphere reduction of cyanometmyoglobin. The second stage proceeds at a rate that is independent of dithionite concentration and corresponds to the dissociation of the transient cyanodeoxymyoglobin intermediate produced in the first stage. The results of the present investigation are compared with those obtained in three independent, previous studies.