Spyros Koinis

The Cr2O72−CrO42−HCrO4− system revisited

Abstract The electronic spectra of aqueous CrVI solutions in the pH range 3–11 have four isosbestic points, located at 246, 293, 338 and 441 nm. These positions are independent of the total concentration of CrVI. The existence of the isosbestic points indicates that only two species are present, namely CrO42− and Cr2O72−, and that HCrO4− is spectrophotometrically undetectable. This conclusion is further supported by factor analysis, which gives the number of linearly independent species in solution to be two, and also by the method of spectral displacement. At the isosbestic points absorbance in linear in total CrVI concentration. In this sense Beers law is valid, and the results can be used for analytical purposes.

Tetrathiomolybdate Complexes of Rhodium(I) with Molybdenum-Rhodium Interactions.

The synthesis and characterization of the tetrathiomolybdatorhodium(I) monoanionic complexes [L2Rh(μ-S)2MoS2](-) (L = CO (3), P(OPh)3 (4), P(O-o-Tol)3 (P(o-CH3C6H4)3; 5), P(OMe)3 (6), P(OEt)3 (7), P(O-i-Pr)3 (8); L2 = COD (1,5-cyclooctadiene; 2), cis-dppen (cis-Ph2PCH═CHPPh2; 9), dppe (Ph2PCH2CH2PPh2; 10), dppb (Ph2P(CH2)4PPh2; 11)) is presented. The complex 2 (NEt4(+) salt) was characterized by X-ray diffraction analysis. A detailed DFT study of the electronic structures of 2-4 and 6-8 has revealed the existence of extended electron delocalization over the four-membered Rh(μ-S)2Mo ring and hence the possibility of electronic communication between the metal centers. The electronic spectra were studied with TDDFT calculations, and the main absorption band in the visible region was assigned to ν(Rh→Mo) electron transfer transition, which is actually a HOMO-LUMO transition. The ν(Rh→Mo) transition was found to correlate linearly both with Tolmans electronic parameter of the phosphite ligands and the calculated HOMO-LUMO gap of the complexes, rendering it a well-defined ligand electronic parameter, which describes the net donating ability of monodentate and bidentate ligands (CO, COD, phosphites, diphosphines). The study of the variation of Δδ((31)P) and (1)J(Rh-P) of the phosphite complexes with respect to the QALE model electronic parameters χd, πp, and Ear has succeeded in the assessment of the σ and π effects on these NMR spectral parameters.

Investigating high-school chemical kinetics: the Greek chemistry textbook and students' difficulties

In this study we present an analysis of how the structure and content of the Greek school textbook approach the concepts of chemical kinetics, and an investigation of the difficulties that 11th grade Greek students face regarding these concepts. Based on the structure and content of the Greek textbook, a tool was developed and applied to students in the form of test questions in order to investigate the difficulties that they encounter with concepts and facts of chemical kinetics. Our results indicate that the textbook presentation at several points does not take into consideration basic findings of chemistry education research, which could improve the understanding of the content, and that a large proportion of Greek students show low comprehension levels of the concepts and facts of chemical kinetics. Given the important role of the school textbook in the learning process, it seems that the structure and content of the textbook do not facilitate the conceptual understanding of the subject matter, and together with other factors contribute to the difficulties that students face regarding concepts in chemical kinetics.