Rigid ligands

The steric crowding introduced in the latter by the four ethyl substituents inhibits nucleophilic attack at platinum, so that complexes of this type tend to undergo substitution by a dissociative mechanism [89]. The complex of the more rigid ligand, 2,2, 2"-terpyridyl, Pt(terpy)Cl+, is found to be about 103 to 104 times more reactive to substitution than the dien analogue this is ascribed to steric strain [90], which is reflected in the short Pt—N bond to the central nitrogen (Pt-N some 0.03 A shorter than the other two Pt-N bonds) and N—Pt—N bond angles of 80-82°). 

Successful chain end control at higher temperatures would require a very rigid ligand environment of the catalytically active center, as on the edges of solid-... 

In the case of complexes with rigid ligands possessing well defined cavities (see also equation III) n in equation VIII has to be replaced by the radius of the ligand cavity rc. 

The flexible macrobicyclie ligands 31—33 display high K+/Na+ selectivity (300—600), but weak K+/Rb+, Cs+ selectivities. On the other hand, the more rigid ligand 30 shows a K+/Na+ selectivity similar to those of the flexible ligands 19 and 31—33, but it also has appreciable K+/Rb+, Cs+ selectivity. 

Fig. 9. Selectivity behaviour of a typical rigid ligand (e. g. 30 peak selectivity) (left), and of a typical flexible ligand (e.g. 33 plateau selectivity) (right)...

The association and dissociation rates are especially slow for the AEC complexes of the relatively rigid ligand 30, which is thus efficiently poisoned by Ba2+ and Sr2+ cations. 

In short, Ni(I) complexes with flexible symmetric ligands such as HTIM and dieneN4 show expansion of the core, while those of rigid ligands in 9,12, and 14 exhibit distortion of the core without an expansion of the hole. The structural characteristics of Ni(I) complexes are summarized in Tables IX and X and compared with those of Ni(II) complexes. 

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