The Effect of Coordination Number

The origin of the relative rates for reductive elimination from complexes writh odd and even coordination numbers. 

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What about the effect of coordination number on rate The iron-sulfur cluster core dendrimers above are four-armed dendrimers. We compared these with... 

The effect of coordination number on the Madelung constant, A, and therefore on the lattice energy, is clearly demonstrated in Table 3.4 when same-formula types are compared the larger the coordination number, the larger the value of A. This is due, of course, to the larger number of attractive interactions that are possible. On the whole, though, this variation amounts to only a few per cent of the lattice energy. 

Table 2.7 Examples of the effect of coordination number and oxidation state (Roman numerals) on ionic radii (pm)...

Factors that influence ionic size include the coordination number of the ion, the covalent character of the bonding, distortions of regular crystal geometries, and delocalization of electrons (metallic or semiconducting character, described in Chapter 7). The radius of the anion is also influenced by the size and charge of the cation (the anion exerts a smaller influence on the radius of the cation). The table in Appendix B-1 shows the effect of coordination number. 

The higher the oxidation state, the stronger the cr-bonding, and the higher the p(CN). The effect of coordination number " is evident in the frequency order ... 

The effect of coordination number on thermodynamic properties is simple to illustrate if one uses the activity of the solvent, o, given by eqn (3.68) ... 

Mn(CO)5] reacts with allyl halides at room temperature to produce the hexacoordinate a-allyl derivative C3H5Mn(CO)5 which must be heated to 60° C before carbon monoxide is lost at an appreciable rate to form the ir-allyl derivative C3HsMn(CO)4 (99,100). In addition, the pentacoordinate perfluoroacyl derivatives of cobalt, RfCOCo(CO)4 (38, 111, 112), lose carbon monoxide to form RfCo(CO)4 at a lower temperature than the hexacoordinate perfluoroacyl derivatives of manganese, RfCOMn(CO)5 (38,113), form the perfluoroalkyl derivatives, RfMn(CO)s. On the basis of present data, however, it is diflicult to separate the effect of coordination number on the general stability of the molecule from the effect of coordination number on the stability of the metal-carbon monoxide bond. 

From the 140-ppm range of these four-connected vertices it is apparent that, in contrast to the effect of coordination number on ("B) in mononuclear species (Section 2.1.1 above), there is no dominant effect of cluster connectivity on in... 

There are a few documented examples of studies of ligand effects on hydrolysis reactions. Angelici et al." investigated the effect of a number of multidentate ligands on the copper(II) ion-catalysed hydrolysis of coordinated amino acid esters. The equilibrium constant for binding of the ester and the rate constant for the hydrolysis of the resulting complex both decrease in the presence of ligands. Similar conclusions have been reached by Hay and Morris, who studied the effect of ethylenediamine... 

Evans and co-workers investigated the effect of a number of -symmetric bis(oxazoline) ligands on the copper(II)-catalysed Diels-Alder reaction of an N-acyloxazolidinone with cyclopentadiene. Enantiomeric excesses of up to 99% have been reported (Scheme 3.4). Evans et al." suggested transition state assembly 3.7, with a square planar coordination environment around the central copper ion. In this scheme the dienophile should be coordinated predominantly in an cisoid fashion in... 

Fig. 13. Calculated effect of coordination number on the ion selectivity of electrically neutral model ligands...

Complexation of Zn11 with ATP results in a significant shift of the H-8 resonance which is taken as evidence of Zn binding to the N-7 atom. The effect of coordination on the C(8)-H bond also explains the formation of a metal-C(8) bond with the MeHg+ ion.120 Multinuclear NMR has been used to identify a number of Al111 complexes of ATP in solution, and the results related to the involvement of Al in dialysis dementia disease.121... 

Sodium and potassium cations are often encountered in the same biological environment and the transmembrane movements of both are required as part of an enzymatic pathway as in Na+, K+-ATPase. Under these circumstances it is essential that cation-specific channels are formed. What features of the channels contribute to the selectivity Earlier the preferred geometries of Na+and K+, sixfold octahedral and eightfold cubic respectively, were proposed as the main discriminatory factors. A computational analysis by Dudev and Lim [35] has considered the effect of coordinated water, number of available coordination sites in the channel walls, and the dipoles of the coordinating groups. The researchers investigated cation complexes with valinomycin and the protein KcsA, both K+-selective, and compared these with a non-selective NaK channel. 

Vesely [93] has expressed the effects of a number of additives in terms of competitive adsorption at the catalyst site by the insertion of a term Ki, [D] in the denominator of eqn, (6). Values of quoted for propene polymerization by a-TiCl3/AlEt3 are given in Table 2. However, if the additives are removed by chemical reaction the use of an equilibrium constant as a measure of their ability to coordinate with the catalytic centre would appear to be a considerable over-simplification. 

The properties of the surplus segment probability p and the effective constraint coordination number z are less well established. It seems possible that p will dep d on polymer species to some extent, since loop projection may be easier for a more locally flexible chain. Weak dependences on concentration and temf rature are likely for the same reason. On the other hand, z characterizes the topology on a fairly large scale and therefore may be virtually a universal constant. Diese however are only some speculations. Values of p and z can be established by various experiments, p from the elastic properties of networks and also from the relaxation of star polymers, z from relative relaxation rates of linear and star molecules in liquids and networks and also from measurements of diffusion rates of stars in linear chain liquids. The adequacy of the... 

The geometry of coordination number 2 would be expected to be linear, either from the point of view of simple electrostatics or from the use of sp hybrids by the metal (but there are exceptions see Chapter 6). If the n — [)d orbitals of the metal are sufficiently close in energy to the ns and np orbitals, the d. orbital can enter into this hybridization to remove electron density from the region of the ligands. The tendency for this to occur will be in the order Hg = Au > Ag > Cu because of relativistic effects- (see Chapter 18). This, in turn, may be partially responsible for the increased softness of Au(I) and Hg(II) (see Chapter 9). 

A large body of data has been collected on substituted methyl silanes, Me4 SiX and Me3 XnSi(CH2)mY. With the exception of coordination number, the greatest effect on 29Si chemical shifts is, of course, caused by substitution changes directly on silicon. The... 

Notably stronger structural relaxation should accompany the formation of sites on the MgO(OOl) surface that are characterized by a reduced coordination number of ions, such as point (vacancy) or extended (step, edge, comer, etc.) defects [87]. This new situation is adequately reproduced by DF cluster models. For instance, for the edge formed by the intersection of (001) and (100) surfaces of MgO, a notable inward (into the substrate) displacement of Mg and O ions from the bulk-terminated position was computed in the [101] direction, by 15 and 12 pm, respectively [95]. Comer three-coordinated Mgsc cations are predicted to move even further, 32 pm, along the [111] direction, "down" to the three O anion neighbors [60], Opposite displacements of the Mg and O ions, to partly restore the bulk-terminated geometry, take place when an adsorbate effectively repairs the reduction of coordination numbers at these defects on clean MgO surface, e.g. Refs. 60 and 95. 

On the basis of some very crude approximations, this paper reports some computations for (1) the phenomena at the melting point, (2) the role of coordination number in liquids, (3) the coefficient of thermal expansion, (4) the effect of pressure on the melting point, and (5) the heat capacity of liquids to corroborate Eyring s theory that liquids can be represented as a mixture of a solid and a gaseous fraction. The results appear to be rather encouraging considering the lack of refinement in the calculations. 

Sigel and co-workers have over a number of years investigated the hydrolysis of a variety of nucleoside 5 -triphosphates and the effects of divalent metal ions. A significant result is that for the hydrolysis of e-ATP (e-ATP is l,A -ethenoadenosine 5 -triphosphate) in the presence of both and Cu +, the most active species is a 2 1 complex metal ion e-ATP (122). It was also proposed that a metal ion bound OH ion effected the hydrolysis. The effect of a number of divalent metal ions on the rates of dephosphorylation of a number of nucleotide triphosphates has also been investigated (128). The ability to dephosphorylate ATP decreased in the order Cu " > Cd > Zn " >Ni >Mn + > Mg-It was suggested that since the pH maximum for the hydrolytic reaction for a particular metal ion paralleled the tendency for that metal ion to form hydroxo complexes (i.e., the p a of the coordinated water molecule) that a metal bound hydroxide ion was involved in the reaction. The most reactive species of the pyrimidine triphosphates could be formulated as [M2(NTP)(OH)J, although the structure of the active complex could only be speculated upon. The reactivity of the purine NTPs was attributed to the dimeric species [M2(NTP)2(0H)]. In both cases, however, two divalent metal ions and a bound hydroxide ion seem to be required to activate the NTPs to hydrolysis. 

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