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The concept of chemical bonding is one of the key concepts in the modern chemical science. The main types of chemical bonds traditionally distinguished in the scientific literature are ionic, covalent and metallic; whereas hydrogen bonds and intermolecular interaction forces are usually taken as secondary bonds. It should be noted, however, that there are no sharp boundaries for the above types of chemical bonds. In order to predict the type of chemical bond between atomic particles, a special atomic property called the electronegativity [i.e. the ability of a given atom in a molecule to attract electron density (or valent electrons)] is widely used. The concept of electronegativity and the first scale of electronegativities of chemical elements were proposed by L. Pauling in 1932; later, several alternative scales of electronegativities were developed, including those by Mulliken and by Allred–Rochow. Using the difference between the electronegativities of atoms (DEN) and the sum of the electronegativities of atoms (SEN), one can obtain quantitative parameters which characterize the chemical bonds in specific compounds. In particular, the difference in the electronegativities of the A and B atoms characterizes the degree of ionicity for the A–B bond, while the sum of electronegativities indicates either the degree of covalency (at relatively large SEN values) or the degree of metallicity (at low SEN values). The sum of electronegativities can be replaced, if necessary, by the average electronegativity (EN av. ) of the compound. Taking the elementary substances Cs and F 2 and the binary compound CsF as reference points in the coordinates EN av. –DEN, a diagram triangle was constructed, which conveniently and clearly illustrates the type of chemical bond of different chemical compounds. The analysis of the ternary chalcogenide A I B V C VI 2 (A I – Li, Na, K, Rb, Cs; B V – As, Sb, Bi; C VI – Se, Te) compounds (which have been studied for a long time by the researchers of the Uzhhorod National University) by using the aforementioned diagram triangle in the coordinates EN av. –DEN has revealed the fact that the considered compounds are characterized by the combined (so-called "metavalent") chemical bonding recently identified in the specialized scientific literature as a separate bonding type.

Сімейство потрійних галогенідів A3B2C9 (A – K, Rb, Cs; B – As, Sb, Bi; C – Cl, Br, I) класифіковано та проаналізовано на основі відповідних даних з літератури по неорганічній хімії та кристалохімії, а також на основі експериментальних результатів, одержаних в лабораторіях кафедри неорганічної хімії Ужгородського національного університету протягом останніх трьох десятиліть.Показано, що властивості вищевказаних сполук, такі як симетрія кристалів, температура плавлення Tпл. і ширина забороненої зони DE, демонструють досить сильні залежності/кореляції від величини їх середнього заряду атомних ядер Zсер.; і тому можна очікувати, що ці властивості можуть будуть ефективно передбачені для ще недосліджених сполук, що належать до тієї ж групи хімічних речовин.Хлориди вищезгаданого типу характеризуються Zсер. £ 31.0, тоді як броміди і йодиди мають значення Zсер. 31.5¸0 42.0 та 43.0¸58.0, відповідно.Встановлено, що хімічне заміщення Cl®Br®I у вищезгаданих потрійних сполуках A3B2C9 призводить до значного зменшення величини DE, тоді як заміщення As®Sb®Bi призводить до збільшення ширини забороненої зони DE для відповідних хлоридів і бромідів. Найвищі значення DE спостерігаються й очікуються для калієвих представників сполук A3B2C9; рубідієві аналоги мають дещо менші значення ширини забороненої зони, тоді як цезієві сполуки мають найменші значення DE. Очікується, що монокристали тернарних галогенідів K3(Rb3,Cs3)Bi2Cl9 з найбільш широкими забороненими зонами DE будуть придатні для використання в якості матеріалів для виготовлення широкосмугових оптичних фільтрів (від інфрачервоного до видимого діапазону спектру); декілька представників розглянутої родини A3B2C9 могли б бути використані як відрізаючі оптичні фільтри.Також було показано, що всі потрійні галогеніди вищевказаного типу A3B2C9 кристалізуються в одному з чотирьох можливих структурних аристотипів; і, як і інші розглянуті властивості, симетрія кристалів цих галогенідів залежить від значень Zсер..Підхід, що використовується в цьому дослідженні, є простим і прямолінійним, тому його можна рекомендувати для рутинних досліджень неорганічних матеріалів, що належать до одного і того ж масиву хімічно і структурно подібних сполук., The family of the ternary halides A3B2C9 (A – K, Rb, Cs; B – As, Sb, Bi; C – Cl, Br, I) have been classified and analyzed based on the relevant literature data on inorganic chemistry and crystal chemistry, and also based on the experimental results obtained in the laboratory of the Department of Inorganic Chemistry of the Uzhhorod National University over the last three decades.It was shown that the properties of the above compounds, such as crystal symmetry, melting point TM and band gap DE, demonstrate reasonably strong dependencies/correlations on their average nuclear charges Zave; so the above properties are expected to be efficiently predictable for the yet uncharacterized compounds belonging to the same family of chemicals.The chlorides the above type are characterized by Zave £ 31.0, while the bromides and iodides have the Zave values of 31.5 ¸ 42.0 and 43.0 ¸ 58.0, respectively.It has been established that the chemical substitution Cl®Br®I in the above ternary A3B2C9 compounds results in a significant decrease of the DE values, whereas the substitution As®Sb®Bi leads to an increase of the DE band gap for the relevant chlorides and bromides. The highest DE values are observed and expected for the K representatives of the A3B2C9 compounds; the Rb analogs have somewhat smaller values of band gap, while the Cs compounds have the smallest DE values. Single crystals of the K3(Rb3,Cs3)Bi2Cl9 ternary halides with the widest band gaps DE are expected to be suitable for use as the materials for manufacturing the wide-band optical filters (from infrared to visible range of the spectrum); a few representatives of the considered A3B2C9 family could be used as optical cullers.It was also shown that all ternary halides of the above A3B2C9 type crystallize in one of the four possible structural aristotype; and, as with the other considered properties, the crystal symmetry of these halides is dependent on the Zave values.The approach used in this study is simple and straightforward, so it can be recommended for routine investigations of inorganic materials belonging to the same array of chemically and structurally similar compounds.

A bactericidal and structural study on the "cetylpyridinium chloride-clinoptilolite" associates has been carried out. For the first time, the samples were synthesized using the powdered clinoptilolite from the Sokyrnytsia deposit (Transcarpathian region, Ukraine). Eight "cetylpyridinium chloride-clinoptilolite" associates have been synthesized using different cetylpyridinium chloride concentrations and further characterized by elemental analysis and FTIR techniques. The association of the cetylpyridinium cation with the zeolite anionic matrix has been studied. It was found that the adsorption of the cetylpyridinium cations occurs on the clinoptilolite surface without destroying the anionic zeolite framework. Furthermore, the antibacterial activity of the associates was analyzed, and a significant activity against Staphylococcus aureus and Candida albicans strains was observed, which is caused by the cetylpyridinium cation presence. Unmodified natural and acid-treated clinoptilolites did not show antibacterial activity; however, these can be effectively used as a matrix carrier for the cetylpyridinium cation.

The solvatochromic effect of solvents on the absorption and fluorescence spectra of three carbocyanine dyes, namely 1,1′,3,3,3′,3′-hexamethylindocarbocyanine chloride (HIC); 1,1′,3,3,3′,3′-hexamethylindodicarbocyanine iodide (HIDC); and 3,3′-diethyloxadicarbocyanine iodide (DODC), have been investigated. The experimental ground and excited-state dipole moments of the dyes have been estimated via the well-known models by Lippert-Mataga, Bakhshiev, Kawski-Chamma-Viallet, and the Dimroth-Reichardt’s empirical model. The influence of the solvation effect on the luminescence quantum yields was also examined and a strong correlation between the luminescence quantum yield values and the solvent viscosity was established. The fluorescence signal enhancement sensitivities between dyes were compared based on the Forster-Hoffmann equation. DFT and TDDFT calculations at the CAM-B3LYP/6-31G(d,p) level of theory were performed for the analysis of the nature of the electron transitions in absorption and excitation processes. Six phases different in polarity were considered during these computations, namely the gas, toluene, chloroform, acetone, methanol, and water. Transition electron density plots, Mulliken atomic partial charges, and Mayer bond orders were used for the analysis of the redistribution of electron density upon excitation. The specific behavior of the dyes in the glycerol medium was interpreted via the analysis of non-covalent interactions between the dye and glycerol molecules in terms of the reduced density gradient method.

A set of effective ionic radii corresponding to different coordination numbers (CNs) and compatible with the radii system by Shannon [Acta Cryst.(1976), A32, 751–767] has been derived for ammonium: 1.40 Å (CN = IV), 1.48 Å (CN = VI), 1.54 Å (CN = VIII) and 1.67 Å (CN = XII). The bond-valence parametersr0= 2.3433 Å andB= 0.262 Å have been determined for ammonium–fluorine bonds.

A calculation scheme based on the bond valence model and involving the distance least squares refinement procedure has been developed for predicting the crystal structures of the ideal ternary oxide pyrochlores.

3,7,8-Trihydroxy-3-methyl-10-oxo-4,10-dihydro-1H,3H-pyrano [4,3-b]chromene-9-carboxylic acid is a structurally well-characterized fulvic acid (FA) capable to act as a polyfunctional bidentate ligand in the complexes with metal ions. Investigations of the formation mechanisms and structure of the above-mentioned FA complexes with bivalent metals [Cd(II), Cu(II), Mg(II), Ni(II), Pb(II) and Zn(II)] are presently an actual and trending topic in the modern chemistry of humic and fulvic acids. Furthermore, the importance of the theoretical DFT investigations of binding of metals with fulvic acids is stipulated by the lack of the relevant experimental structural data for such complexes. The quantum chemical calculations have shown that, of the four possible FA tautomers, the two FA forms are more stable. The wavefunction analysis and computed reactivity descriptors (electrostatic potential, Hirshfeld surface analysis, natural population analysis charges, and condensed Fukui indexes) give the insight on the properties and reactive ability of these two different forms of the FA. The computed thermochemical parameters of the ion-exchange reaction explain the metal binding affinity and selectivity of the FA forms.

Using the multiple linear regression technique, an efficient empirical model based on the weighted ionic radii has been developed for predicting the cell parameters of the high symmetry argyrodite structures crystallizing in the cubic space group F-43 ​m (No. 216) and having no strictly stoichiometric chemical compositions.

A new thiazolylazo structure, 1-[(5-(3-nitrobenzyl)-1,3-thiazol-2-yl)diazenyl]naphthalen-2-ol (NBnTAN), has been synthesized for the first time. The geometric parameters and properties of the new dye were studied using spectroscopic, DFT and semiempirical methods through the computation of different electronic descriptors (aromaticity, frontier molecular orbitals, dipole moments, partial atomic charges). The solvation effect of different organic liquids on the absorption spectra of the reagent has been studied. 1H, 13C, COSY, HSQC and HMBC NMR, UV–Vis and IR spectra were used to confirm the reagent structure. It has been shown that NBnTAN forms complex compounds with a number of transition metals, allowing the development of selective methods for the determination of these metals.

A few different theoretical methods for assigning the partial atomic charges were benchmarked for calculation of the hydrophilic/lipophilic index (HLI). The coefficients were selected to produce the best correlation of the HLI values with the experimental octanol-water partition. Different parameters were checked in calculations of partial charges to get the best performance of the HLI values obtained. Thus, four partitioning schemes (Coulson, Mulliken, Merz-Kollman, Ford-Wang) were benchmarked for calculations of atomic charges with six semiempirical methods (AM1, PM3, RM1, PM6, PM6-D3H4, PM7). Moreover, five distinct types of partial atomic charges (Mulliken, Hirshfeld, Löwdin, CHELPG, NPA), obtained at the Hartree-Fock and DFT levels of theory with three basis sets, were tested for their ability to produce the HLI values with the best correlation to experimental logP coefficients of 50 mono-charged organic anions. In the case of the semiempirical methods, the best correlation between the HLI and logP values (the correlation coefficient r = 0.9216) was obtained with the AM1 Ford-Wang parametric electrostatic potential charges. The Mulliken and Coulson charges calculated with the PM7 method can be used as an alternative to AM1, with the r values of 0.9107 and 0.8984, respectively. In the case of the DFT, the PBE/def2-TZVP natural population analysis charges produce the best correlation (r = 0.9220). Nevertheless, in spite of a marginally lower performance (r = 0.9159), the NPA charges computed at the PBE/def2-SVP level are more robust and can be regarded as the optimum choice for calculating the HLI values. Graphical abstract The hydrophilic/lipophilic index (HLI).

Cu7PS6 crystals were grown using direct crystallisation from the melt (Bridgman–Stockbarger technique). The crystal structure of Cu7PS6 was determined by X-ray powder diffraction. Cu7PS6 crystallise in the cubic structure, space group P213 (No. 198), the lattice parameter a = 9.6706(1) Å, formula units per cell Z = 4. Electrical properties of Cu7PS6 crystals were studied in the frequency range 10–1010 Hz and in the temperature interval 296–351 K. The performed analysis shows that the Cu7PS6 crystals are mixed electron-ionic conductors with some growth defects.

The oxidation-state independent `bond valence (s)versusbond length (r)' correlation curve for manganese–oxygen bonds has been closely approximated using the modified two-parameter Trömels=f(r) function [Trömel (1983).Acta Cryst.B39, 664–669],s= [(r0−l)/(r−l)]2, wherer0= 1.763 (2) Å andl= 1.148 (9) Å. Ther0andlrefinable parameters of the above function can be regarded as the alternative bond-valence parameters intended for use in the modern bond-valence model [Brown (2009).Chem. Rev.109, 6858–6919] in cases where the traditional bond-valence parameters (r0;n) and (r0;b) fail.

The crystal structure of Sb 3 O 6.5 has been refined using the Rietveld analysis of X-ray powder diffraction data. The refinement procedure was stopped when the intensity residual R B = 6.1% had been reached. Sb 3 O 6.5 crystallizes in a cubic structure of the defect pyrochlore type, space group Fd -3 m (No. 227), with the lattice parameter a = 10.3024(5) A, Z = 8, D c = 5.701(1) g/cm 3 .

The crystal structure of the ternary iodide Cs2TeI6 has been refined by Rietveld analysis of X-ray powder diffraction data. The refinement procedure was stopped when the intensity residual RB = 3.7 % had been reached. Cs2TeI6 crystallizes in the cubic K2PtCl6 structure type, space group Fm-3m (No. 225), with the lattice parameter a = 11.6939(8) A, Z = 4, Dc = 4.797(1) g/cm . The reliability of the structural model obtained for Cs2TeI6 in the Rietveld refinement was confirmed by a bond-valence analysis of the structure. For the first time, the bond-valence parameters (r0 = 2.782 A and b = 0.37 A) have been determined for the Te /I ion pair, from a set of 14 well-determined coordination shells [TeIn]. These parameters show a reasonably high performance and can be recommended for routine bond-valence analyses of structures containing Te–I chemical bonds.

The crystal structure of Hg2Sb2O7 has been refined using X-ray powder diffraction technique. Hg2Sb2O7 crystallizes in the cubic pyrochlore structure type, space group F d 3 ¯ m , with the lattice parameter a = 10.3525(5) A, Z = 8, Dc = 9.060(1) g/cm3. The Rietveld refinement procedure was stopped when the intensity residual RB = 3.25% had been reached. In the bond-valence analysis of the crystal structure of Hg2Sb2O7, the values of the bond-valence parameters reported for the Sb5+/O2− ion pair have been found to be doubtful. Using the new calculation scheme, the improved bond-valence parameters for the Sb5+/O2− ion pair (r0 = 1.908 A and b = 0.409 A) have been derived from the crystal structure of Sb2O5.

The relationship between the bond valencesand the thermal expansion rate of chemical bonds (dr/dT) has been closely approximated by using the alternative three-parameter empirical model (dr/dT) = (u+vs)–1/w, whereu,vandware the refinable parameters. Unlike thes–(dr/dT) model developed by Brownet al.[(1997),Acta Cryst.B53, 750–761], this alternative model can be optimized for particulars–(dr/dT) datasets in the least-squares refinement procedure. For routine calculations of the thermal expansion rates of chemical bonds, the alternative model with the parametersu= −63.9,v= 2581.0 andw= 0.647 can be recommended.

The crystal structures of Tl 2 TeBr 6 and Tl 2 Tel 6 have been determined using X-ray powder diffraction techniques. Tl 2 TeBr 6 crystallizes in the tetragonal space group P4/mnc, with the lattice parameters: a = 7.468(1) A, c = 10.682(2) A, Z = 2. Tl 2 TeI 6 crystallizes in the monoclinic space group P2 1 /c, with the lattice parameters: a = 7.765(2) A, b = 8.174(2) A, c = 13.756(5) A, β = 124.2(1)°, Z = 2. The Rietveld refinement procedures were stopped when the intensity residuals R B = 0.038 for Tl 2 TeBr 6 and R B = 0.049 for Tl 2 TeI 6 had been reached. The crystal structures of both investigated ternary halides belong to the large family of K 2 PtC 6 -related structures.

The compound Rb3Bi2I9 was synthesized by melting a stoichiometric mixture of RbI and BiI3 and annealing for 1 month some degrees below the melting temperature of 785±5K; a large single crystal of the title compound has been grown from the melt by directional solidification using the vertical Bridgman-Stockbarger technique. X-ray structure determination of the compound involved single-crystal diffractometry, direct and Patterson methods and least-squares refinement. Rb3Bi2I9 exhibits a new type of crystal structure, crystallizing in the monoclinic space group Pc with unit-cell parameters a=14.690(4), b=8.195(2), c=25.645(6) A, β=125.39(2)°, V=2517(2) A3, Z=4. The unit-cell of Rb3Bi2I9 has six pseudohexagonal RbI3 layers which are stacked along the direction near to the normal of the XY plane. Each BiI6 octahedron shares three corners with three other octahedra to form parallel Bi2I9 corrugated layers intersecting Z axis of the structure at an angle near 120°.

The calculation scheme for determining the bond-valence parameters (r 0 and b) resulting in the exact solution of the bond-valence sum rule for a given set of coordination shells is presented.

Using the least-squares refinement procedure, high-performance Brown–Wu bond-valence parameters, r 0 and n, reported for the Sn2+/O2−, Sb3+/O2−, Te4+/O2− and I5+/O2− ion pairs have been converted into the commonly used bond-valence parameters, r 0 and b. The r 0 and b values obtained can be recommended as a replacement for the conventional bond-valence parameters based on the `universal constant' b = 0.37 Å.

Systematic variations of the bond-valence sums calculated from the poorly determined bond-valence parameters [Sidey (2008), Acta Cryst. B64, 515–518] have been illustrated using a simple graphical scheme.