Structural Interconversion

In this section we shall mention the few studies which have been concerned with structural interconversion at labile metal ions. They have been involved with cobalt(ii) and nickel(n), although it might be mentioned that several groups of workers have apparently been unable to repeat the previously reported study on the octahedral tetrahedral transformation of hydrated Co + and Zn +. 

The kinetics of octahedral tetrahedral conversion in nitromethane-pyridine have been reported. The mechanism apparently involves a five-co-ordinate intermediate  

The number of chemists interested in the problem of ion transport through cellular membranes has increased markedly in recent years, and the tendency to approach the problem from the molecular standpoint has developed to a large degree because of some remarkable observations with model compounds, such as those reported in the November-December, 1968 issue of Federation Proceedings. It has been found that certain macrocyclic antibiotics, such as nonactin, enniatin, and valinomycin, 

In view of these and other theories, it is important to answer questions concerning the relative affinities of the various antibiotic molecules for different metal ions, the conformations of the macrocycles before and after formation of the ion complexes, the rates of formation and dissociation of the complexes, and so on. In the remainder of this section we shall consider recently published work on this very exciting topic, the starting point being -ray crystal structure determinations of the various complexes. 

Pinkerton, Steinrauf, and Dawkins have determined the structure of the potassium complex of valinomycin (3). They, also, found that the 

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C. Cluster Structural Interconversions and Synthesis of Heterometal Clusters... 

We have examined the effects of concentration, temperature, solvent and added electrolyte on the kinetics of this structural interconversion. In all instances, the kinetics are well described by the rate law for a reversible first-order reaction [Equation 1] ... 

This contribution reviews recent results on [Si(Pc)0]n (Pc = phthalocyaninato) solid state electrochemistry and the structural interconversions that accompany electrochemical doping/undoping processes. In aceto-nitrile/(a-Bu)4N+BF4, it is found that a significant overpotential accompanies initial oxidation of as-polymerized [Si(Pc)0]n. This can be associated with an ortho rhomb ic- te tr agonal structural transformation. 

However, if the nature (e.g. electronic configuration) of the first singlet excited state. S, and the first triplet excited state 7 , are known, the electron-transfer is easy to handle in a mechanistic sense. An important preequisite is to know if the Si and/ or 7 , are the exclusive precursor states for the electron-transfer product or if intermediates such as upper excited states, biradicals or ground-state intermediates might be involved. In addition, structural properties have to be considered as well because the states of a chemical system are parametrized in terms of distinct structures. For instance, not all structural interconversions are allowed under a given set of reactions conditions (e.g. solvent or temperature) [88]. 

An interaction of metal ions with phosphates usually leads to stabilization of polynucleotide secondary and tertiary stmctures, depending on the metal ions and their concentrations, whereas base binding or chelating base and phosphate by the same metal ion may result in destabilization, conformational change, or denaturation of the polynucleotide structures (see also Sections 3.3.1 and 3.3.2). However, phosphate-specific binding can also induce significant conformational changes in DNA structure. The mechanisms of the latter structural interconversions are... 

Isotropic shift measurements of some triazene-1-oxide complexes of nickel(ii) show appreciable contact shifts. (124) A detailed study of the temperature dependence of the isotropic shifts of octahedral Ni(ii) complexes with a variety of amino and amide ligands reveals apparent non-Curie behaviour for most shifts. (125) The authors consider the likely causes of this anomalous behaviour, namely, pseudocontact shifts, ion-pair formation, hydrogen bonding, solvent elfects, structural interconversion, and temperature independent paramagnetism. 

Two Co(ll) coordination polymers with formulas [Co(oda)(H20)2 H20]n and [Co(oda)(H20)-H20]n (H2oda=oxydiacetic acid) were characterized by single crystal XRD and TG. [Co(oda)(H20)2 H20]n has a covalently linked 1-D chain structure, while [Co(oda)(H20) H20]ii has a covalently linked 3-D chiral network with channels. The structures showed an unusual example of topological isomerism, and the structural interconversion between [Co(oda)(H20)2 H20]n and [Co(oda)(H20)-H20]n revealed that self-assembly in the synthesis and interconversion of crystalline solids is a thermodynamically controlled process [135]. 

The use of cyanidometallate linkers between SCO metal centres has also generated a range of interesting porous phases. One 3D example, [Fe (pmd)(H20)(M (CN)2)2]-H20 (pmd = pyrimidine = Ag, An) displays the multifunctional properties of SCO, hysteresis (AT = 8K) and a reversible dehydration/rehydration structural interconversion in... 

Figure 16 Reversible structural interconversion between HgeLg complex 22 and Hgel cage 23.

This reaction system is classified as Type II because coordination polymers with large channels and no channels are reversibly switched depending on the present solvents, which function as chemical stimuli. For this conversion, the coordinations of the solvent molecules, H2O and DMF in this case, to Zn(II) centers at the first step would largely contribute to switch the structures. Similar structural interconversions have been observed in Zn-oba system by using mechanochemical method. In this system, PCP [Zn2(bdc)2(H20)2] DMF and non-PCP [Zn(bdc)(H20)2] are switched by grinding the sample in the presence of small amount of solvent. ... 

One-dimensional coordination polymer [Ni(dps)2(N03)2]-EtOH (dps = 4,4 -dipyridylsulfide) (6a) has shown a unique structural interconversion, which is classified as Type IV, in response to temperature variations. " This structural conversion produced the closed channels at lower temperature, where the guest molecules are trapped mechanically. 

As pointed out earlier, the process of structural interconversions while retaining the same numbers of nuclei (in the same overall oxidation state) is accomplished by the... 

Rearrangements, it will be recalled, involve nonrandom structural interconversions in which the same number of nuclei is present in the substrate and the product but their connectivity has changed. For example, in addition to undergoing SnI and... 

Structural interconversions of dichloro-l,l,7,7-tetraethyldiethylenetriamine-nickel(n) (NiLClg) in acetonitrile have been studied by relaxation methods in which the equilibria were suddenly perturbed by an electric-field jump or by means of a pulse of radiation from a Q-switched neodymium laser. The results are interpreted in terms of the mechanism... 

A novel slant on the relative stabilities of axial and equatorial conformers in six-membered rings has been provided by the use of diamantan-l-ol (14) and diamantan-4-ol (15). In (14) hydroxyl is axial with respect to the heavy cyclohexane ring whereas in (15) it is equatorial Although the normal ring-inversion modes are precluded by the cage structure, interconversion of (14) and (15) is readily effected. Alcohols (14) and (15) were equilibrated in 98% sulphuric acid at seven temperatures between 273 and 473 K at higher temperatures small amounts of other products were formed. At temperatures above 321 K axial isomer (14) is thermodynamically more stable, notwithstanding... 

NbO and TiO [21]. Here the square planar geometry is stabilized by the presence of jr-acceptor Nb (or Ti) d orbitals. Likewise, molecules in which 02u is empty are expected to be more stable at the square planar rather than tetrahedral or C4V pyramidal geometry. Examples include BH4, CH4 +, or the doubly excited state of CH4 in which 02u is empty and b g is filled [22]. We will return to this problem of tetrahedral-square planar-C4v pyramidal structure interconversion in Chapter 14, where AL4 molecules with more than eight electrons are considered. 

First, it is necessary to assume that the process of structural interconversion is not a general phenomena, but is restricted to a narrow range of related triterpenoids. Thus,... 

The structure of the salen ligand system has been modified by using a dipyrrin -instead of the ethylenediamine-unit. The novel pentacoordinated dipyrrin-silicon complexes (31, see Scheme 4, and some of its derivatives) showed efficient red or near-IR fluorescence, and the structural interconversion between silanol and siloxane derivatives resulted in significant changes in the optical properties [150]. 

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