Halogens cationic complexes

The reactivity of the initial halogen bonded complex has also received considerable attention. Husebye and coworkers have suggested a process whereby the dihalogen bond is cleaved to give a key cation intermediate and a halide anion (Eq. 4) [182]. This mechanism is consistent with that often proposed for nitrogen electron donor systems. 

A relatively stable aqua complex or protonated hypoastatous acid [H20At] has been assumed similarly, a protonated hypoiodous acid has been reported to exist in aqueous solutions (15). The equilibrium constant for the deprotonation reaction [Eq. (9)] has been estimated by extrapolation of data accrued from the lighter halogens to be < 10" (80), indicating that [H20At]" is a fairly weak acid. Another structure, the symmetric diaqua cationic complex [H20-At-0H2]", has also been proposed (79, 80). 

The conventional preparative routes to anionic, neutral, or cationic complexes of indium start with the metal, which is dissolved in a suitable mineral acid to give a solution from which hydrated salts can be obtained by evaporation. These hydrates react with a variety of neutral or anionic ligands in nonaqueous solvents, and a wide range of indium(III) complexes have been prepared in this manner.1 Alternatively, the direct high-temperature oxidation of the metal by halogens yields the anhydrous trihalides, which are again convenient starting materials in synthetic work. In the former case, the initial oxidation of the metal is followed by isolation, solution reaction, precipitation, and recrystallization. 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

The skeletal nitrogen atoms in cyclophosphazenes possess a lone pair of electrons and, hence, they have long been viewed as potential donor sites to bind a proton or to form complexes with electron-acceptor molecules. The possibility of formation of anion-cation complexes by release of a halogen ion to a Lewis acid and charge-transfer complexes has also been studied. In addition, some cyclopho-sphazene derivatives form crystalline inclusion clathrates with a variety of guest molecules. Allcock (21, 22) has reviewed these aspects in detail. 

The cationic complexes [Ir(CO)2(SbPh3)3]+ and [Ir(CO)2(PR3)2]+ (R = Ph, C6Hn) have been synthesized from iridium carbonyl halide derivatives with halogen acceptors in the presence of CO.109... 

These complex anions are very weakly basic, i.e. of very low nucleophilicity, and have been used deliberately as counter-ions in the isolation of solid compounds where the cation is highly electrophilic and subject to disproportionation or other reaction with species of reasonable base strength. Relative basicities of anions can be of great importance in isolating such compounds. An example which will be discussed in detail in Sec. 11.3.4.6 involved attempts to synthesise solid compounds of IJ and BrT. These were unsuccessful when the counter-ion was fluorosulfate, because of disproportionation of the halogen cations, whereas the cations were stable in the presence of less basic fluoro-antimonates. 

Using a somewhat different approach, Fischer and Fichtel (85) have obtained a variety of ethylene-containing cationic complexes by reaction of TT-cyclopentadienyl carbonyl halides of Mo, W, and Fe with ethylene under pressure in the presence of aluminum chloride. The cations were isolated in the form of their salts with heavy anions such as PFe , B(C6H5)4, etc. Presumably the aluminum chloride serves as a halogen acceptor and activates a coordination site on the metal, facilitating coordination of ethylene. 

Cr(CO)2(diars)2 and Cr(CO)3(l-triars) (l-triars = bis(3-dimethylarsinopropyl)methylar-sine) are oxidized by molecular halogens to the seven-coordinate cationic complexes [Cr(CO)2(diars)2X] (X = Br, I) and [Cr(CO)3 (l-triars) ], respeetively. The oxidation of Cr(CO)3 (r-triars) [r-triars = bis(o-dimethylarsinophenyl)methylarsine] by halogens is solvent dependent in benzene the seven-coordinate cation, [Cr(CO)3(i -triars)I] forms, while in CHCI3, the complex Cr(u-triars)X3 (X = Br, I) is obtained. 

Note that the reaction proceeds with inversion of configuration at the stereogenic center attached to the metal. It is not clear exactly what the initial function of the halogen is in the reaction, however. Scheme 8.7 presents two possible scenarios for the reaction. In path a, the halogen oxidatively adds to the metal to give a cationic complex. The remaining nucleophilic halide ion attacks the complex at the... 

The product thus obtained undergoes oxidative addition of halogens, leading to unusual tetranuclear species, which give cationic complexes by substitution of the halogen atoms by a neutral ligand (Figure 8)199-202. 

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