Complexes anion centred

In all the cluster compounds discussed above there are sufficient electrons to form 2-centre 2-electron bonds between each pair of adjacent atoms. Such is not the case, however, for the cationic bismuth species now to be discussed and these must be considered as electron deficient . The unparalleled ability of Bi/BiCb to form numerous low oxidation-state compounds in the presence of suitable complex anions has already been mentioned (p. 564) and the cationic species shown in Table 13.12 have been unequivocally identified. 

The stracture of BaS04 itself, and our interpretation of it, is shown in Fig. 3. The Ba -f- S array is that of a well-known alloy stracture, FeB(B 15), shown in Fig. 4. Thus, in terms of the complex ion SO4 , the stracture may be described as FeB type with SO4 in place of B and Ba in place of Fe (as in the centre of Fig. 3) or, better, in terms of anion-centred polyhedra it is simply a BaS array of B15 type with the oxygens inserted into SBa3 tetrahedra. These anion-centred tetrahedra are irregular - as is to be expected, because the sizes of Ba and S are different. 

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). 

The major reagents used in the study of both small and large molecules have been either Ln(III) aquo cations or Ln(III)(FOD)3 complexes54. It has been found that Ln(III)(EDTA) complexes are valuable reagents for anionic centres and that... 

The complex anion [PtCl4]2- has a centre of symmetry. What shape is it ... 

The reagent NSF3 may also react via oxidative addition of an S-F bond to a metal centre to give a complex of the thiazyl difluoride anion [NSF2] , which is readily hydrolyzed to an NSO complex (Scheme 7.3). ... 

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). 

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