Luis A. Gentil

On some bivalent transition- and post-transition metal nitroprussides

Abstract The following hydrated nitroprussides are described: Mn[Fe(CN) 5 NO]·2H 2 O, Fe[Fe(CN) 5 NO]·4H 2 O, Co[Fe(CN) 5 NO]·6H 2 O, Ni[Fe(CN) 5 NO]·4H 2 O, Cu[Fe(CN) 5 NO]·2H 2 O, Zn[Fe(CN) 5 NO]·3H 2 O, and Cd[Fe(CN) 5 NO]·2H 2 O. Spectroscopic and magnetic properties are reported and discussed on the basis of the structures assigned to these compounds. Comparisons are made with previously reported results of some of these and also on related hydrates. The anhydrous compounds are also dealt with briefly.

The crystal and molecular structure of sodium hexacyanoosmate(II) decahydrate and related hexacyanometalate complexes

The crystal structures of sodium hexacyanoosmate, ruthenate and ferrate decahydrates, Na4M(CN)6·10H2O(M=Os, Ru, Fe), have been determined from X-ray diffraction data and refined by full matrix least-squares to final agreement values: R = 0.038, Rw = 0.039; R = 0.026, Rw = 0.041; R = 0.060, Rw = 0.043 for Os, Ru and Fe compounds, respectively. The compounds are isostructural and crystallize in the monoclinic space group P21/n, Z = 2, with a = 9.154, b = 11.506, c = 9.876 A, β = 97.95°; a = 9.146, b = 11.486, c = 9.867 A, β = 98.00°; a = 9.038, b = 11.450, c = 9.782 A, β = 97.57°, for Os, Ru and Fe compounds, respectively. The structure can be described as layers of hexacyanometallate anions, intercalated with layers of sodium polyhedra containing hydration water molecules and N atoms, perpendicular to the crystallographic ac plane. MetalC and CN distances for the hexacyanide anions are correlated with those from other structurally related moieties. The infrared spectra of the compounds are complementary with previous results for potassium salts.

Nitrosyl–nitrite interconversion in pentacyanoruthenate(II) complexes

The nucleophilic addition of OH– to [Ru(CN)5(NO)]2– leads to the N-bonded pentacyanonitroruthenate(II) ion (λmax= 320 nm, Iµ= 3850 dm3 mol–1 cm–1). The stoichiometry is 2:1 ([OH–]:[Ru]) and the equilibrium formation constant is 4.4 × 106 dm6 mol–2(25 °C, I= 1 mol dm–3). A kinetic study of the forward reaction showed that it is first order in the concentration of each reactant, with k= 0.95 dm3 mol–1 s–1(25 °C, I= 1 mol dm–3), ΔH‡= 57.3 ± 3.3 kJ mol–1 and ΔS‡=–54.0 ± 4.5 J K–1 mol–1. The mechanism involves two consecutive attacks by OH–, the first being rate determining. The reaction product decays by an aquation process, leading to [Ru(CN)5(H2O)]3– and free NO2–. The rate constant for the dissociation reaction of [Ru(CN)5(NO2)]4– is k–N= 2.00 × 10–4 s–1(25 °C, I= 1 mol dm–3). In the formation reaction, both nitrite (O-bound) and nitro (N-bound) linkage isomers are formed, with ko and kN being 0.23 and 0.15 dm3 mol–1 s–1 respectively (25 °C, I= 1 mol dm–3). The O-bound isomer isomerizes slowly to the thermodynamically more stable N-bound isomer. The kinetic and thermodynamic parameters have been analysed by comparison with the chemistry of the complexes [Fe(CN)5(NO)]2– and [Fe(CN)5(NO2)]4–.

Das thermische Verhalten von Na5[Fe(CN)5SO3]·2H2O

The thermal behaviour of Na5[Fe(CN)5SO3]·2H2O has been investigated by thermogravimetric and differential thermal analysis. Detailed studies of the decomposition residues using various physicochemical methods show that decomposition occurs in a manner similar to that previously postulated for related compounds.

Preparation, characterization and substitution kinetics in the complexes of pentacyanoruthenate(II) with hydrazine

Abstract Potassium pentacyanohydrazineruthenate(II) trihydrate has been synthesized and characterized by chemical methods, IR and electronic spectroscopy. The specific rate constants for the formation, k L , and dissociation, k− L , reactions of the [Ru(CN)5L]n− ions (L = N2H4, N2H5+) were obtained, as well as the corresponding stability constants, K L . A value of pKa for the bonded N2H5+ species was also calculated. The results were interpreted in terms of dissociative-type mechanisms for both reactions, and comparisons were made with the related [Fe(CN)5L]n− system.

Kinetics of hydrazine exchange in the pentacyanohydrazineferrate(II) ion

SummaryRate constants and activation parameters for the release of hydrazine (both neutral and protonated) coordinated to the Fe(CN)53− moiety are reported, together with the corresponding stability constants. The rates of formation of the complexes have been evaluated indirectly. The results confirm the general dissociative mechanism for pentacyano(ligand) ferrate(II) ions but some significant differences are found on comparing the data with those from several coordinated diamines. The acidity of complexed N2H5+ is enhanced byca. two orders of magnitude with respect to free N2H5+, its rate of release being greater than that of N2H4 by a factor of 31. A solvation controlled model for the dissociation of [Fe(CN)5L]n− is shown to be operative in its general features, although specific charge effects are shown to influence the rate of both the dissociation and the formation reactions.

Electronic d-d spectra and ligand-dissociation rates of pentacyanoruthenate(II) ions

The electronic spectra of a series of [Ru(CN)5L]n– complexes (L = H2O, NH3, pyrazine, dimethyl sulphoxide, or CN–) have been measured in aqueous solution. The d-d bands were assigned on the basis of a ligand-field model for tetragonally distorted octahedra and the energy of the transitions, which depends on the nature of L, were found to correlate with the rate constant for the dissociation of L from [Ru (CN)5L]n–, k–L. Comparisons are made with the similar [Fe(CN)5L]n– series. A new value of 10Dq, 4.2 µm–1, is proposed for [Ru(CN)6]4–, which is significantly higher than that corresponding to [Fe(CN)6]4–, as expected from ligand-field theory.

Ligand substitution kinetics in complexes of pyridine and pyrazine derivatives with pentacyanoruthenate(II)

The reactivity of the pentacyanoruthenate(II) complexes of derivatives of pyridine and pyrazine (L) are reported. The kinetics of dissociation of the complexes, measured in the presence of an excess of a scavenging ligand, show saturation behaviour with respect to this reactant. The limiting rates, k–L, are in the range (2–10)× 10–5 s–1(25 °C). A dissociative interchange mechanism is postulated, as in related pentacyanoferrate(II) complexes. The rates of formation, starting from [Ru(CN)5(H2O)]3–, are typically first order with respect to the concentration of the ligands, with k=ca. 1 dm3 mol–1 s–1(25 °C) for neutral ligands and slightly higher and lower values for positively and negatively charged ligands, respectively. An ion-pair dissociative interchange mechanism is postulated. Activation parameter data are presented for both types of reactions and compared with those from related systems, [Fe(CN)5L]n– and [Ru(NH3)5L]n+.

Kinetics of formation, dissociation, and isomerization reactions in complexes of cyanopyridines and benzonitrile with pentacyanoruthenate(II)

The complexes of pentacyanoruthenate(II) with 3- and 4-cyanopyridines and benzonitrile have been prepared in aqueous solution by mixing [Ru(CN)5(H2O)]3– with the appropriate ligand. With the cyanopyridines, the reaction proceeds in two stages: the first corresponds to the formation of a mixture of pyridine- and nitrile-bonded isomers, and the second to the linkage isomerization of the unstable (nitrile-bonded) isomer to the stable one (pyridine-bonded). The complexes exhibit metal-to- ligand and intra-ligand charge-transfer absorptions. The rate constants for the formation, kf, and dissociation, kd, of all the complexes have been determined (25 °C), as well as the isomerization rate constant, kiso, for the cyanopyridine complexes. Activation parameters were obtained for the formation reaction of the mixture of isomers, as well as for the dissociation reactions of the stable, pyridine-bonded complexes. The results for the formation and dissociation reactions are analysed in terms of dissociative mechanisms. From kf and kd, values of Kst, the stability constants of the different complexes, have been calculated. The discussion is supported by previous data obtained for related pentacyanoferrate(II) and penta-ammineruthenium(II) complexes. Thus, the influence of the metal centre, M, as well as that of the auxiliary ligands, X, on the spectral and substitution reactivity properties of MX5L systems is described.

Das Thermische Verhalten von Na3[Fe(CN)5SO(CH3)2] · H2O

The thermal behaviour of Na3 [Fe(CN)5SO(CH3)2] · 2H2O has been investigated by thermogravimetry and diferential thermal analysis. The results are briefly discussed.