Cationic complexes group 4 metals

These phenomena can be explained by the (hard-soft) acid-base principal as follows C=N-OH is a soft base, hence has stronger affinity towards soft basic metal cations than hard metal cations. The strong participation of the N-OH group in complex formation was further confirmed by the results shown for extraction experiments with 5 and 6. 

EDTA forms 1 1 complexes with a large number of cations, including those of some of the main-group metals. The complex formed by calcium with EDTA is used to treat lead poisoning. When a... 

Macropolycyclic ligands, 2,942 classification, 2,917 metal complexes binding sites, 2, 922 cavity size, 2,924 chirality, 2, 924 conformation, 2,923 dimensionality, 2, 924 electronic effects, 2, 922 shaping groups, 2,923 structural effects, 2,922 molecular cation complexes, 2,947 molecular neutral complexes, 2,952 multidentate, 2,915-953 nomenclature, 2,920 Macro tetrolide actins metal complexes, 2,973 Macrotricycles anionic complexes, 2,951 cylindrical... 

Humic substances can form complexes with metals, including cationic micronutrients (36), thanks to the presence of electron-donor functional groups in these molecules. It therefore appears evident that due to these properties, humic substances can contribute to the regulation of the chemical balances of metals, thus influencing their solubility (5). With regard to plant availability, the molecular dimension and solubility of humic substances are very important. 

Studies of the base-hydrolysis mechanism for hydrolysis of technetium complexes have further been expanded to an octahedral tris(acetylacetonato)techne-tium(III) [30], Although a large number of studies dealing with base hydrolysis of octahedral metal(III) complexes have been published [31], the mechanism of the tris(acetylacetonato)metal complex is still unclear. The second-order base hydrolysis of the cationic complex tris(acetylacetonato)silicon(IV) takes place by nucleophilic attack of hydroxide ion at carbonyl groups, followed by acetylacetone liberation, and finally silicon dioxide production [32], The kinetic runs were followed spectrophotometrically by the disappearance of the absorbance at 505 nm for Tc(acac)3. The rate law has the following equation ... 

Although transition metal alkylidene complexes, i.e., carbene complexes containing only hydrogen or carbon-based substituents, were first recognized over 15 years ago, it is only relatively recently that Ru, Os, and Ir alkylidene complexes have been characterized. Neutral and cationic complexes of these Group 8 metals are known for both metal electron configurations d8 and d6. The synthesis, structural properties, and reactivity of these compounds are discussed in this section. 

Terminal methylene complexes are relatively rare—less than 10 such compounds have been isolated and about as many again have been characterized by spectroscopic techniques only. The methylene complexes previously reported fall into two groups, (i) neutral complexes of the early transition metals (e.g., Ti, Ta) and (ii) cationic complexes of the later transition metals (e.g., Re, Fe). The osmium complex 47 is important, then, as it is a new example extending the neutral group to the later transition metals. Compound 47 is the prototype for the series Os(=CHR)Cl(NO)(PPh3)2 and is one of only three terminal methylene complexes to be structurally characterized by X-ray crystallography (see Section IV,B). 

Eisch s work promoted investigation into the preparation of cationic metallocene complexes of Group 4 metals. Several preparative routes to cationic group 4 metallocene complexes are illustrated in Scheme II. Catalytic activities of some selected cationic metallocene complexes for the polymerization of a-olefins are summarized in Tables 5 and 6. The catalyst systems based on these cationic complexes are just as active as M AO-activated metallocene catalysts for the polymerization of a-olefins. 

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

Surprisingly, the polymerization rate has practically a zeroth-order dependence on the concentration of the monomer, which is a rare example for a group 4 metal-based catalyst. Although the reason for the zeroth-order dependence is unclear at the current time, one possible explanation is that, under the conditions examined, the cationic complex virtually exists as a (higher a-olefm)-coordinated form, presumably due to the highly electrophilic and sterically open nature of the cationic active species. 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

---