Abbas A. El-Awady

Semiempirical molecular orbital calculations: The iron tricarbonyl polyene aldehyde isomers☆

Abstract Semiempirical all valence electrons extended Huckel molecular orbital calculations of the charge self consistent type were performed on the possible adducts obtained from the interaction of 9-methyl-2,4,6,8-nona-tetraenal and iron tricarbonyl. The organic moiety offers three different butadiene fragments, and the stability of the possible isomers obtained is predicted. In addition the electron densities on the various atoms of the molecules were calculated, and hence their susceptibility to substitution reactions is analyzed.

Displacement of chelate ligands from planar four-co-ordinate complexes. Part I. Reactions of some gold(III) complexes of 2,2′-bipyridyl

The kinetics of nucleophilic displacement of the chelate 2,2′-bipyridyl ligand (bipy) from the complex [Au(bipy)X2]+ by X–(X = Cl or Br) have been measured in aqueous methanol solution at 25 °C. The rate constant shows dependences on hydrogen-ion concentration and on the first and third power of chloride-ion concentration. Details of the mechanism are discussed and compared with others involving replacement of chelate ligands from planar complexes of d8 transition-metal ions.

Kinetics of isomerization of αβR-isothiocyanatotetraethylenepentaammine-cobalt(III) cation

Abstract The isomerization of αβR -Co(tetren)NCS 2+ to the αβS isomer has been studied spectrophotometrically in 0·1–1·0 F HOAc and 0·2–1·0 F NaSCN (μ 1·4 M, NaClO 4 ) at 50–70°. The observed first-order rate constant k obs. has the form k obs. = k 1 + k 2 [SCN − ] at 0·10 F HOAc; k 1 has a small hydrogen ion dependence. In 0·10 F HOAc at 25° (by extrapolation), k 1 = (3·0±0·3) × 10 −8 sec −1 and k 2 = (4·6±0·9) × 10 −9 M −1 sec −1 ; for the thiocyanate-independent ( k 1 ) path, E a = 34·5±0·9 kcal mole −1 and log PZ (sec −1 ) = 15·0±0·5, and for the thiocyanate ( k 2 ) path, E a = 34·5±0·9 kcal mole −1 and log PZ (M −1 sec −1 ) = 16·7±0·6. The isomerization probably occurs via a proton-exchange mechanism involving the secondary amino group common to the two coplanar chelate rings.

Kinetics of the acid dissociation of cyclic and open-chain tetramine complexes of cobalt(II): general acid catalysis with co-ordinating phosphate—citric acid buffers

The kinetics of dissociation of the cobalt(II) complexes of the quadridentate ligands 1.4.8.11-tetraazacyclotetradecane (cyclam), triethylenetetramine (trien) and 2,2′,2″-triaminotriethylamine (tren) was followed spectrophotometrically in the ranges 10 < T < 50 °C and 0.8 < pH < 4.0 in perchloric acid and Mcllvaine phosphate–citrate buffer system, an an ionic strength I= 1.0 mol dm–3, NaClO4. The complexes were prepared in situ in preaerated solutions under a nitrogen atmosphere, by the addition of a 10% excess of the ligand in the form of the free base to a solution of CoCl2·6H2O. The ligand dissociation reaction was then initiated by the addition of perchloric acid or Mcllvaine phosphate-citrate buffer. No dissociation was observed for the cyclam complex in perchloric acid media. It was, however, observed in the buffer and obeyed biphasic kinetics comprising two consecutive first-order steps. The action of the phosphate and/or citrate is attributed to their complexing ability and to their association through hydrogen bonding to the axial aqua ligand, thus bringin the proton closer to the dissociating nitrogen and hence catalysing the dissociation. The dissociation kinetics of the open-chain unbranched trien and the tripod tren was observed in perchloric acid as well as the Mcllvaine buffer system. Except when too fast to follow by conventional spectrophotometry, the reaction obeyed biphasic kinetics comprising two consecutive first-order steps. The tren complex dissociates at a rate 5–10 times faster than that of trien, the first step being too fast to follow except at 10 °C in the buffer system. The observed rate dependence is explained on the basis of mechanisms involving solvation, specific acid catalysis and general acid catalysis. The cyclam complex dissociates at a rate 5–30 times slower than those of the two open-chain complexes. This is attributed to the stabilization due to the hindered rotation of the dissociating nitrogen (entropic effect) and to the higher ligand-field stabilization of the macrocycle (enthalpic effect). Mechanisms covering the entire range of pH studied and conforming to the observed rate laws are given.