Complexes cation centred

This corresponds to a decreased energy of the interaction between cation and anion caused by an increased level of anion complexation. When one takes the atomic charges of the cationic centre (carbon atom) into consideration, the improvement of the anion properties becomes clearer (see Fig. 9). 

Let us now examine the consequences of the formation of a donor-acceptor bond in a little more detail. If the donor - acceptor bond is completely covalent, then we record net transfer of one unit of charge from the donor to the acceptor as a direct consequence of the equal sharing of the electron pair between the two centres. This result leaves a positive charge on the donor atom and a negative charge on the acceptor atom. The limiting ionic and covalent descriptions of a complex cation such as [Fe(H20)6] are shown in Fig. 1-1. 

The P— y-Ca2Si04 transformation is of some considerable interest, as it may be accompanied by dusting - a serious matter in cement, of which Ca2Si04 is a constituent. There is doubt as to whether the transformation is reconstructive or displacive. In one paper" it is decided that it is semi-reconstructive in another no decision was reached. Comparison of the two structures involved is, in conventional terms involving emphasis on cation-centred coordination polyhedra, a complex task. It would appear to be much simpler in terms of the approach adopted in the present paper. 

From the beginning it was noted that the structure was dominated by large columnar elements of the garnet structure type The doubt has been about the remaining parts (between the garnet columns). The garnet structure itself is rather complicated it has only recently been given a satisfactory description in terms of articulated, cation-centred polyhedra The vesuvianite structure is even more complex. 

Cation-centred complexes as found in the example above are those most often encountered, but anion-centred building blocks also exist in, for example, H2O and OPb . The latter complex is stabilized by its stereoactive lone pair (Section 8.2) which allows a strong Pb-0 bond of 0.50 vu to form with the central 0 anion. The remaining bonds formed by Pb " " are weaker and serve to link the OPb complex with neighbouring anions (Krivovichev and Filatov 1999). 

Figure 2.19 The molecular structure of the [Cu2,(17>212 1 double-strand helicate complex cation. Cu1 metal centers are represented as spheres. Hydrogen atoms of the two strands have been omitted for clarity. Structure redrawn from data deposited at the Cambridge Crystallographic Data Centre CCDC 641164.

A number of polymer films related to PVP are known. An obvious variation is polybipyridyl. Thus the complex cation tris(4-vinyl-4 -methyl-2,2 -bipyridyl)ruthenium(II) may be electroreduced in acetonitrile solution to give a polymer film on a platinum electrode. Pulsing of the electrode at 0.5 Hz between +1.5 and —1.5 V (vj. SCE) gives an orange emission (eel) arising from annihilation between RuII and Ru1 centres.59... 

In the case of inert cobalt(m) complexes it is possible to isolate the chelated products of the reaction. Let us return to the hydrolysis of the complex cations [Co(en)2(H2NCH2C02R)Cl]2+ (3.1), which contain a monodentate iV-bonded amino acid ester, that we encountered in Fig. 3-8. The chelate effect would be expected to favour the conversion of this to the chelated didentate AO-bonded ligand. However, the cobalt(iu) centre is kinetically inert and the chloride ligand is non-labile. When silver(i)... 

This liberates two potential coordination sites which are occupied by labile solvent in the resting state of the catalyst. This particular case is slightly more complex than simple alkenes in that further coordination of the amide group to the cationic centre enhances the enantioselectivity. 

The molybdenum compound with the empirical formula MoClg does not possess the simple Mo + ion its structure is [MogClg]4Cl4, the complex cation being cubic, with chlorine atoms at the eight corners and molybdenum atoms at the six face centres (Fig. 244). Tungsten forms a similar compound but, unlike the Mo complex which is stable in aqua regia, it is easily oxidised. 

The emerging EPR active superoxide anion species O2 is complexed by a cation (which must not necessarily be a constituent of the anion vacancy) and interacts now with the IR active OH group previously involved in the H bond with the colour centre electron [22,25], The gzz signal component of O2 is sensitively influenced by the local crystal field of the complexing cation [31,32], On the other hand the OH stretching frequency depends on the strength of the interaction of OH with O2, Thus the redox reaction described by eq,6 may be monitored by EPR and IR spectroscopy. In Fig,5 the respective curves are compared for fundamentally different experimental conditions ... 

It is assumed, however, that complex cation species such as FeCL, VO(OH), MoCl4 or CrOo + with extra-framework ligands are located on caiion positions rather Uian Fe +, V , Mo or Cr . This is suggested [33] because of the large distance between the framework A1 centres in the highly dliceous zeolites, e.g. in the H-ZSM-5 zeolite used (Si/Al=35). 

Ion-pair formation in the system. In systems containing complex cations, for example, the solvating effect and relative permittivity of the solvent control the extent of ion-pair formation between the complex cation and the associated anion similarly to the effect of hydrogen bond formation, this may lead to a change in, or the formation of, a dissymmetry centre,... 

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