Yuriko Abe

A fluorescent EXAFS study on the structure of the solvated cobalt(II) ion and chlorocobalt(II) complexes in hexamethylphosphoric triamide

Abstract The coordination structure of the cobalt(II) ion and its chloro complexes of low concentration (∼20 mmol dm −3 ) in hexamethylphosphoric triamide (HMPA) has been explored by laboratory EXAFS (extended X-ray absorption fine structure) modified for fluorescence detection. The cobalt(II) ion is four-coordinated in HMPA, unlike other oxygen-donor solvents such as water and N,N -dimethylformamide (DMF). The CoO (HMPA) bond lengths within complexes are revealed to be 194(1), 195(1), 194(1) and 202(2) pm for [Co(hmpa) 4 ] 2+ , [CoCl(hmpa) 3 ] + , [CoCl 2 (hmpa) 2 ] and [CoCl 3 (hmpa)] − , respectively. The CoO bond length is significantly shorter than that within six-coordinate [Co(H 2 O) 6 ] 2+ (208 pm) or [Co(dmf) 6 ] 2+ (206 pm), and remains practically unchanged upon formation of mono- and dichloro complexes. On the other hand, the CoCl bond length is 224(1), 227(1) and 229(1) pm within [CoCl(hmpa) 3 ] + , [CoCl 2 (hmpa) 2 ] and [CoCl 3 (hmpa)] − , respectively, all of which are appreciably shorter than that within the six-coordinate [CoCl(H 2 O) 5 ] + (235 pm). The origin of the different thermodynamic behavior of chloro complexation of cobalt(II) in HMPA from that in DMF will be discussed on the basis of these structural data.

Unusual thermodynamic behaviour on complexation of cobalt(II) with chloride, bromide and iodide ions in hexamethylphosphoric triamide

The complexation of cobalt(II) with chloride, bromide and iodide ions has been studied by spectrophotometry and calorimetry in hexamethyl-phosphoric triamide (HMPA) containing 0.1 mol dm–3(n-C4H9)4NCIO4 as a constant ionic medium at 25 °C. The formation of [CoXn](2–n)(n= 1–4 with X as CI, n= 1–3 with X as Br and n= 1 and 2 with X as I) is proposed, and their formation constants, enthalpies and entropies are determined. Electronic spectra of individual cobalt(II) halogeno complexes are also extracted, indicating that the cobalt(II) ion is four-coordinated in HMPA(unlike other oxygen-donor solvents which show six-coordination), and four-coordinate tetrahedral [CoXn(HMPA)4–n](2–n)+(n= 1–4) are formed stepwise. In spite of the remarkable strong donicity of HMPA compared to other aprotic donor solvents, the ΔH°1values of formation of [CoX]+ are even smaller, or more negative, in HMPA is observed in contrast to endothermicity in N,N- dimethylformamide or dimethyl sulphoxide. This suggests that, although being sterically allowed, the four-coordination of [co(HMPA)4]2+is considerable crowded for bulky HMPA molecules, and Co—O(HMPA) bonds are thus elongated to some extent.

Solvation and halogeno complexation of the cadmium(II) ion in hexamethylphosphoric triamide

The complexation of the cadmium(II) ion with bromide and iodide ions has been studied by calorimetry in hexamethylphosphoric triamide (HMPA) containing 0.1 mol dm–3(n-C4H9)4NClO4 as a constant ionic medium at 25 °C. The formation of [CdXn](2–n)+(n= 1, 2, 3, 4 for X = Br; n= 1, 2, 3 for X = l) is revealed, and their formation constants, reaction enthalpies and entropies were determined. The ternary CdII–Br–I system has also been examined and the formation of the [CdBrI]0 and [CdBr2I]– complexes is elucidated. The CdII–X systems exhibit unusual thermodynamic behaviour that is not observed in the MII–X (M = Co, Zn) systems. The reaction enthalpies very in the order, ΔH°1 > ΔH°2≈ΔH°4 > ΔH°3 in the CdII–Br system, while ΔH°1 < ΔH°2 < ΔH°3 < ΔH°4 in the ZnII–Cl system. The ΔS°1 values, 179 and 187 J K–1 mol–1 for the formation of [CdBr]+ and [CdI]+, respectively, are significantly larger than the corresponding values in the CoII and ZnII systems. These facts strongly suggest that the cadmium(II) ion has a larger solvation number in HMPA. EXAFS spectroscopy provided evidence that the cadmium(II) ion is coordinated to five HMPA molecules, whereas cobalt(II) and zinc(II) ions are coordinated to four HMPA molecules.

Complex of cobalt(II) bromide with hexa­methyl­phospho­ric tri­amide

The title compound, dibromobis[tris(dimethylamino)phosphine oxide]cobalt(II), [CoBr2(C6H18N3OP)2], displays tetrahedral coordination about cobalt. The molecule has twofold crystallographic site symmetry. The short P-N bonds and the planarity of the dimethylamino groups indicate the importance of dpi-ppi interactions. One of the NMe2 groups has an irregular conformation about the P-N bond and deviates from planarity. It is ascribed to the steric hindrance induced by coordination at the O atom of hexamethylphosphoric triamide.

Thermodynamics of tetrahedral-octahedral configurational interconversions in CoII complexes in nitromethane

Abstract The visible absorption spectra for [Co(hmpa)4](ClO4)2-L-nitromethane systems (L = H2O, MeOH, EtOH, n-PrOH, i-PrOH, n-BuOH, FA, NMF, DMF, DEF, DMA, DEA or DMSO) have been studied. The results indicate that the configurational inter- conversions from the tetrahedral [Co(hmpa)4]2+ to the octahedral [Co(hmpa)2L4]2+ accompanying the ligand substitution reactions occur in nitromethane, except for the DMA and DEA systems. The values of both ΔH and ΔS for the interconversions increase in the following order: H2O

X-Ray Diffraction Study of the Solvation Structure of the Cobalt(II) Ion in N,N-Dimethylformamide Solution

Abstract The solvation structure of Co2+ in N,N-dimethylformamide (DMF) has been studied by X-ray diffraction measurements on cobalt (II) and magnesium (II) Perchlorate solutions of the same concen tration, using an isostructural substitution method. The radial distribution function revealed three distinct peaks assigned to the oxygen, amido carbon (C1, and nitrogen atoms of six planar DMF molecules in the first coordination sphere around the metal atom. The distance from the cobalt atom to each atom (O, C1, N) is 213,299, and 423 pm, respectively. This indicates that the Co-O-C1 bond angle is 122-123° and the metal atom is close to the O-C1-N plane of the DMF molecule.

Base hydrolysis in homometallic dinuclear chromium(III) complexes bridged by hydroxide and carboxylate

Base hydrolysis of dinuclear complexes [Cr2(µ-OH)(µ-RCO2)(en)4]4+ and [(nta)Cr(µ-OH)(µ-RCO2)Cr(en)2]+(en = ethane-1,2-diamine, nta = nitrilotriacetate; R = H, Me, Et, Prn, CH2Cl, CH2ClCH2, MeOCH2 or Ph) was studied by using the UV/VIS absorption and 2H NMR spectral changes and chromatography. The reactions were found to occur in two stages: the first-stage rates (k1) varied with R, but the second-stage ones (k2) were almost the same throughout the series. Analysis of the kinetic data for the two types of complexes disclosed the effects of the substituent groups R and/or the non-bridging ligands on the carboxylato bridge-cleavage reaction mechanism. Examination of the substituent effects in terms of Tafts equation with the electronic (ρ*) and steric (δ) parameters gave a critical clue to establishing the difference in bond-cleavage sites. The values were ρ*= 2.29 ± 0.20 and δ= 1.41 ± 0.13 for [Cr2(µ-OH)(µ-RCO2)(en)4]4+ and ρ*= 0.01 ± 0.11 and δ= 0.32 ± 0.08 for [(nta)Cr(µ-OH)(µ-RCO2)Cr(en)2]+. The carboxylato bridge-cleavage reaction is influenced by both the inductive and steric effects of the substituents in [Cr2(µ-OH)(µ-RCO2)(en)4]4+, and both effects contribute to smaller extents for [(nta)Cr(µ-OH)(µ-RCO2)Cr(en)2]+; the former complexes undergo cleavage at the carboxylic acyl C–O bond and the latter at the Cr–O bond in the Cr(en)2 site.