The bond-valence method, especially the valence-sum rule, is very useful for checking if the structures formed by trivalent lanthanides are correct. In this work bond-valence parameters (Rij), which connect bond valences and bond lengths, have been computed for a large number of bonds taken from the Cambridge Structural Database, Version 5.24 (2002) [Allen (2002). Acta Cryst. B58, 380-388]. The calculated values of bond-valence parameters for metal-organic compounds decrease with an increase in lanthanide atomic number; the Rij values are also smaller than bond-valence parameters calculated for inorganic compounds. A summary of bond-valence sums calculated for Rij given in this work and reported in the literature, and a functional correlation between lanthanide-oxygen distances and coordination number are presented.

The bond-valence parameters (R(ij)), which connect bond valences and bond lengths, have been computed for lanthanide-nitrogen bonds. It has been found that values of bond-valence parameters decrease with increasing lanthanide atomic number in coordination compounds, and that they are smaller than the R(ij) parameters of inorganic compounds. As expected, the lanthanide-nitrogen bond-valence parameters are larger than lanthanide-oxygen bond-valence parameters. There are no obvious dependencies between the number of N atoms in the coordination sphere and the bond-valence parameter value.

The impact of highly electron donating units appended to the imine ligand on the thermal and optoelectronic properties of Re(i) complexes was investigated.

Systematic study of π-π interactions of structurally characterised compounds containing parallel benzene and/or pyridine rings was carried out. The gathered geometrical parameters were analysed in statistical terms. The quantum mechanical calculations were made for the above mentioned ring systems in different arrangements and the calculated interaction energy values were referred to the statistical data. The maximum bonding energy of the studied systems is about 3 kcal/mol. The ring rotation about the vertical axis has almost no influence on the system binding energy. In the specific ring arrangements, the stacking interactions can be bonding even for ring centroids distances larger than 6 Å. The results prove that the appliance of the generally accepted geometrical criteria of stacking interactions leads to the omission of the multiple bonding intermolecular interactions during the interpreting of the reactivity, self assembly as well as the properties of the supramolecular compounds. AbstractSystematic study of π-π interactions of structurally characterised compounds containing parallel benzene and/or pyridine rings was carried out. The gathered geometrical parameters were analysed in statistical terms. The quantum mechanical calculations were made for the above mentioned ring systems in different arrangements and the calculated interaction energy values were referred to the statistical data. The maximum bonding energy of the studied systems is about 3 kcal/mol. The ring rotation about the vertical axis has almost no influence on the system binding energy. In the specific ring arrangements, the stacking interactions can be bonding even for ring centroids distances larger than 6 Å. The results prove that the appliance of the generally accepted geometrical criteria of stacking interactions leads to the omission of the multiple bonding intermolecular interactions during the interpreting of the reactivity, self assembly as well as the properties of the supramolecular compounds.

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The preparation and spectroscopic and structural characterization of three cobalt(II) complexes of formulas [Co(tppz)](dca) (1), [Co(tppz)][Co(NCS)]·MeOH (2), and [Co(tppz)][Co(NCO)]·2HO (3) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide] are reported here. Compounds 1-3 have in common the presence of the cationic [Co(tppz)] entity where each mer-tridentate tppz ligand coordinates to the cobalt(II) ion equatorially through two pyridyl donors and axially via the pyrazine, completing the six-coordination. The electroneutrality is achieved by the organic dca group (1) and the anionic tetrakis(thiocyanato-κN)cobaltate(II) (2) and tetrakis(cyanato-κN)cobaltate(II) (3) complexes. Direct current (dc) magnetic susceptibility measurements of 1 in the temperature range 1.9-400 K show the occurrence of a thermally induced spin crossover behavior of the [Co(tppz)] unit from a high spin (S = 3/2) at higher temperatures to a low-spin (S = 1/2) at lower temperatures, with the low spin phase being reached at T ≤ 200 K. X-band electron paramagnetic resonance (EPR) measurements in solution at low temperatures were used to characterize the low spin state. An analytical expression based on the combination of the spin-orbit coupling and both first- and second-order Zeeman effects for a d electronic configuration was used to fit the magnetic data of 1, the values of the best-fit parameters being C = 0.1367(9), λ = -168(2) cm, α = 1.12(1), Δ = 1626(15) cm, and g = 2.12(1). The magnetic behavior of the four-coordinate cobalt(II) ions [Co(NCS)] (2) and [Co(NCO)] (3) with a A ground state overlaps with the spin crossover of the [Co(tppz)] entity, the abrupt decrease of the χT product below 15.0 K being due to zero-field splitting effects between the spin components |±1/2> and |±3/2>. The combined analysis of the dc magnetic data and the Q-band EPR spectra in the solid state of 2 and 3 led to the following sets of best-fit parameters: C = 0.105(5), λ = -170(4) cm, α = 1.10(2), Δ = 1700(25) cm, g = 2.10(1), g = 2.27(1), and |D| = 3.80(2) cm (2) and C = 0.100(1), λ = -169(5) cm, α = 1.10(3), Δ = 1500(30) cm, g = 2.10(1), g = 2.28(1), and |D| = 4.30(2) cm (3). Some evidence of slowing of the relaxation of the magnetization has been found in the out-of-phase ac signal at very low temperatures under applied dc fields of 0.1-0.4 T for 3, suggesting the occurrence of single-ion magnet behavior of its [Co(NCO)] anionic entity.

Ln-O and Ln-N bond-valence parameters have been computed in coordination complexes for lanthanides (Ln) at oxidation states other than +3 (Ce(IV), Sm(II), Eu(II) and Yb(II)). Moreover, Ln-Cl, Ln-S and Ln-C(pi-bonded) bond-valence parameters are presented, as calculated for coordination compounds. In general, the bond-valence parameters decrease in the order Ln-O > Ln-C > Ln-N > Ln-Cl > Ln-S. It has been found that the values of bond-valence parameters decrease with increasing lanthanide atomic number for coordination compounds. As expected, the values of lanthanide-oxygen and lanthanide-nitrogen bond-valence parameters diminish with increasing lanthanide oxidation state. Several examples are given where the total valence of the lanthanide ion is apparently incorrectly assigned, as well as cases where bond-valence method calculations confirm the doubtful oxidation state assignment.

The discovery of endogenous opioid peptides with their limited receptor selectivity more than two decades ago implicated their involvement in analgesia and initiated efforts to understand the molecular mechanisms underlying their action. Opioid peptides mediate their physiological and pharmacological effects through three major opioid receptor types (mu, delta, kappa), but the role of each of these receptors is not easy to distinguish. There has therefore, been an increasing need for potent and selective agonists and antagonists in this area. The incorporation of conformational constraints into analogs of biologically active peptides is a well-known approach for enhancing receptor selectivity and modulating efficacy. Conformational restriction has proven a valuable means for assessing disparities between the peptide binding characteristics at each of the opioid receptor types, since peptide analogs designed with appropriate conformational constraints possess the ability to adopt the conformation required for interaction at one receptor type but not another. In efforts to obtain better conformational integrity various conformationally restricted analogs of endogenous opioid peptides have been developed. In this paper we review the development of opioid analogs whose conformation was restricted either globally through different types of cyclization (such as amide bond formation, disulfide and monosulfide bridges, carbonyl and amine bridges) or locally, through incorporation of side-chain conformational constraints at a specific amino acid residue by synthesizing cyclic amino acids or constraining torsion angles by suitable substitutions. These two approaches have led to the development of potent and very selective agonists and antagonists at all three opioid receptor types.