Structural equilibria

Calculated AG( — 50°Q values are in the range 38-6 to 43-7 kJ mol and rate constants in the range 11 x 10 to 4-9 x 10 s . Similar studies on other Ni(ii) complexes are reported (54,242,243,252). Co(ii) complexes with ethyl phosphite, phosphonite, and phosphinite ligands (L) may be high spin complexes of the type C0CI2L2 or low spin, 5-coordinate complexes of the type C0CI2L3. In organic solvents the equilibrium  

The degree of non-rigidity of these complexes depends greatly on the nature of the metal. For example, the Rh(R,R -dtc)3 complex is rigid up to +200°C, the Cr(R,R -dtc)3 complex up to 84°C, whereas vanadium(iii) and manganesefin) complexes show averaged spectra by — 20 and — 45°C respectively. Typical kinetic parameters are given in Table XI. The overall metal ion dependence on the rate of optical inversion is In,Ga,V Mn Fe Ru Co Rh, with Cr at least less than Ru. The values for Fe(iii) complexes depend also on the position of the spin equilibrium S = S = f (see below). 

Pseudo-octahedral complexes of Ni(ii) with 1,3-diamine chelates have been shown (247) to undergo a rapid conformational interconversion [46]. In the diagram, (a) refers to the chair-chair equilibrium and (b) to the twist-twist equilibrium. The results indicate that 

The conformations of Ni(ii) complexes with ethylenediamine-N,iV -diacetic acid type quadridentate ligands [47] have been examined by 

NMR and CD methods. (251) From the spectral data it is concluded that these complexes stereospecifically adopt the A-s-cis form(s) in solution [48]. The solution structures of a variety of EDTA-type complexes have been deduced from variable temperature studies. (306) Racemization of Ni(EDTA) and Ni(l,3-DDTA) is rapid at 80°C but is slow for the other complexes studied. The kinetic data indicate predominant 5-coordination by EDTA and 1,3-PDTA. The suggested racemization mechanism (Fig. 5) involves a 7-coordinate intermediate. 

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Chemistry can be divided (somewhat arbitrarily) into the study of structures, equilibria, and rates. Chemical structure is ultimately described by the methods of quantum mechanics equilibrium phenomena are studied by statistical mechanics and thermodynamics and the study of rates constitutes the subject of kinetics. Kinetics can be subdivided into physical kinetics, dealing with physical phenomena such as diffusion and viscosity, and chemical kinetics, which deals with the rates of chemical reactions (including both covalent and noncovalent bond changes). Students of thermodynamics learn that quantities such as changes in enthalpy and entropy depend only upon the initial and hnal states of a system consequently thermodynamics cannot yield any information about intervening states of the system. It is precisely these intermediate states that constitute the subject matter of chemical kinetics. A thorough study of any chemical reaction must therefore include structural, equilibrium, and kinetic investigations. 

Application of isotropic shifts to the investigation of structures and structural equilibria of metal complexes. R. H. Holm, Acc. Chem. Res., 1969, 2, 307-316 (38). 

Substituent effects have fascinated organic chemists for generations and their study is still an active area of research. The generalization of the influence of substituents is expected to lead to an understanding of physical properties, structures, equilibria and reactions in organic chemistry (Schleyer, 1987). Substituents can be considered as perturbations of a given standard system and it is often believed that their character remains basically unaltered from one molecular situation to another, i.e. an invariable universal nature of a substituent is assumed. 

The Influence of Nudeobase Methylations on Secondary Structure Equilibria I 11... 

The anthocyanins are structurally dependent on the conditions and composition of the media where they are dissolved and suffer interactions among them and with other compounds that influence their structural equilibria and modify their color. Anthocyanins are usually represented as their red flavylium cation, but in aqueous media this form undergoes rapid proton transfer reactions, leading to blue quinonoidal bases, and hydration, generating colorless hemiketals in equilibrium with chalcone structures. The proportion of each form is determined by the pH... 

Keywords silicocations, synthesis, structure, equilibria, trityl salts... 

In this book an attempt is made at summarizing developments of the last decade and presenting the material in combination with essential information established earlier. Emphasis is placed on mechanistic aspects and applicability in organic synthesis. The individual chapters deal with the oxidation of 11 broader classes of organic compounds. As transition metal dioxygen complexes are usually though not invariably involved in these oxidations, a chapter on the major features of their structure, equilibria and formation kinetics has also been included. 

NUCLEAR RESONANCE STUDIES OF STRUCTURAL EQUILIBRIA IN SOLUTIONS OF xNICKEI/m COMPLEXES. 

Complexes of the bis(salicylaldimine) series have been prepared in which the nitrogen substituent R carries a functional group such as "OCH, -OH, -N(CHj)2. These complexes are shown to exist as octahedral complexes in the crystalline state, but in solution are involved in observably temperature dependent structural equilibria amongst octahedral, tetrahedral, and planar forms with the proportion of tetrahedral species increasing with increasing temperature. The magnetic susceptibilities, optical spectra, and proton resonance contact shifts are analyzed in terms of these equilibria. 

Chemistry is concerned with the study of molecular structures, equilibria between these structures and the rates with which some stractures are transformed into others. The study of molecular structures corresponds to study of the species that exist at the minima of multidimensional PESs, and which are, in principle, accessible through spectroscopic measurements and X-ray diffraction. The equihbria between these structures are related to the difference in energy between their respective minima, and can be studied by thermochemistry, by assuming an appropriate standard state. The rate of chemical reactions is a manifestation of the energy barriers existing between these minima, barriers that are not directly observable. The transformation between molecular structures implies varying times for the study of chemical reactions, and is the sphere of chemical kinetics. The journey from one minimum to another on the PES is one of the objectives of the study of molecular dynamics, which is included within the domain of chemical kinetics. It is also possible to classify nuclear decay as a special type of unimolecular transformation, and as such, nuclear chemistry can be included as an area of chemical kinetics. Thus, the scope of chemical kinetics spans the area from nuclear processes up to the behaviour of large molecules. 

Delocalization of c -electron spin from a transition metal has been used to study structure, equilibria, and reactions of complexes, particularly ligand exchange and solvation. Other applications are to organometallics, such as metallocenes, metallobiomolecules such as porphyrins (haemoglobins, cytochromes), iron-sulfur clusters, and so on. Organic radicals such as nitroxides or aromatic radical ions are useful spin probes also. The field of paramagnetic NMR spectroscopy has been reviewed biennially. ... 

Selzer, T. and Rappoport, Z. (1996) Triphenylethenethiol. Structure, equilibria with the thioketone, solvation, and association with DMSO. J. Org. Chem., 61, 5462-5467. 

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