D-Structure of Aqueous Solutions of Cobalt(II) Nitrate at 298 and 323 K by X-Ray Diffraction

Abstract

Aqueous solutions of cobalt(II) nitrate with 1 : 15, 1 : 25, and 1 : 40 mole ratios at 298 and 323 K were studied by the X-ray diffraction. As the solution concentration decreases, ion triads (1 : 15) pass into contact ion pairs (1 : 25) and then to independently hydrated ions (1 : 40). The structural variations are accompanied by increase in a number of water molecules in the second coordination sphere of the cations and in the hydration shell of the anions. Heating produces destruction of ion triads in the 1 : 15 solution and ion association in the 1 : 40 solution. The structure of all the solutions at 323 K is characterized by a system of contact ion pairs. The structure of concentrated aqueous solutions of electrolytes attracts considerable attention of researchers involved in the fundamental and applied science. Correlations of the trend in structure formation in aqueous electrolyte systems with the chemical nature of ions and solution concentration have been revealed [1]. Recently polythermal studies of such solutions have gained special importance [234]. To obtain statistical data on quantitative characteristics of aqueous electrolyte solutions and to extend the body of experimental information, in the present work we performed an X-ray diffraction study of aqueous solutions of cobalt(II) nitrate at various mole ratios and temperatures. The Co + ion is a complex-forming ion, and a series of X-ray researches have been devoted to its complex formation in solutions of cobalt(II) chlorides and bromides. The aim of the present work was to study structural aspects of ion hydration in solutions, and, therefore, we chose nitrate ions as counterions not inclined to complex formation.In early works [5, 6] on the basis of measuring viscosity of aqueous CoSO 4 solutions it was noted that Co 2+ ions reinforce water structure in a wide concentration range. Kinetic studies on oxygen exchange between nitrate ions and water molecules in aqueous solutions of Co(NO 3 ) 2 at concentrations close to the boundary of complete hydration revealed a twolayer hydrate surrounding [7,8]. In the crystal hydrates Co(NO 3 ) 2 . 2H 2 0 and Co(NO 3 ) 2 . 4H 2 0, cobalt ions are surrounded by six oxygen atoms at distances of 0.205130.2124 nm, of which two atoms belong to nitrate ions (0.206030.2097 nm) and the other two in the first case and four in the second, to water molecules [9]. The octahedral coordination of water molecules around the cation in concentrated aqueous solutions of cobalt(II) halides [10312] and sulfate [13] was revealed by X-ray diffraction analysis, the Co 2+ 3 OH 2 distances varying within 0.20730.216 nm. The formation of contact ion pairs by counterions in CoCl 2 solutions of various concentrations was established [11,14]. Aqueous solutions of CoCl 2 : nH 2 O (n = 6, 4, 2) at various temperatures (298, 313, 328, and 343) were studied by X-ray diffraction. In octahedral cobalt(II) complexes, the Co3O and Co3Cl distances were found to be 0.211 and 0.240 nm, respectively [15]. The number of Co3Cl contacts in saturated...

Revisiting the cobalt(II) hydration from molecular dynamics and X-ray absorption spectroscopy

“…The value for the Co-OI distance is similar to the value provided by the EXAFS fit, and within the range of values proposed in the literature. The Co-OII distance is shorter than the theoretical values previously proposed,[12,15,16] in the range 4.1-4.6 Å, but larger than the experimentally reported from X-ray diffraction measurements, 4.05 Å [9]. The depletion zone between the first and second peak indicates there are no water exchanges between these hydration shells during the simulation time.…”

“…[12,15,16] in the range 4.1-4.6 Å, but larger than the experimentally reported from X-ray diffraction measurements, 4.05 Å. [9] The depletion zone between the first and second peak indicates there are no water exchanges between these hydration shells during the simulation time. The running integration numbers are 6 and ∼ 15 for the first and second shell, respectively.…”

“…[5] An infrared study together with a combined electronic spectroscopy and multi-reference quantum mechanical study support the six-fold coordination as the most representative aqua ion structure in solution. [6,7] X-ray diffraction studies [8,9] proposed a first-shell Co-OI distance range of 2.00-2.10 Å and Levochkin et al [9] a second hydration shell centered at 4.05 Å. EXAFS fittings found hexacoordination for the Co(II) aqua ion and a Co-OI range of 2.06-2.09 Å with Debye-Waller factors in the range 0.0038-0.0062 Å2 . [10][11][12] D'Angelo et al [13], from the fitting of the XANES spectrum, proposed a hexahydrated ion with the Co-OI distance equals to 2.06 Å.…”

NO3–Coordination in Aqueous Solutions by15N/14N and18O/natO Isotopic Substitution: What Can We Learn from Molecular Simulation?

“…The hydration behavior (speciation) of nitrate ion in aqueous solution is of central interest in the development of potential selective extractants for nitrate ion in technological applications, and in understanding its reactivity and eventual fate in natural ( e.g. , groundwater) environments. , Consequently, it has been the target of numerous experimental studies involving X-ray, − neutron diffraction, − nuclear magnetic resonance (NMR), − Raman, ,,,− ,,− and infrared (IR) spectroscopies. − The experimental effort has been complemented by theory and modeling involving quantum and ab initio methods, − as well as molecular simulations ,− to aid the elucidation of the aqueous environment around the anion, its dynamics, and the effect of the nature of the counterion.…”

Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump

“…Steady-state, optical, and x-ray absorption studies (9,10) ; remaining charge compensation comes from the added chloride salt, in our case LiCl. The first three species have an octahedron configuration, whereas the latter two adopt a tetrahedron configuration because of the crystal field stabilization energy (9,11). The striking spectroscopic shift from pink to dark blue when increasing [Cl Ϫ ] or temperature is due to the shift of equilibrium from the octahedral to the tetrahedral structure, each with a characteristic absorption.…”