Borate complexes cation effects

Speculation is made about why epi-inositol flips into the triaxial form to form a tridentate complex with borate, but complexes without flipping with metal ions. The distances between the three axial oxygen atoms are similar to those between the three oxygen atoms in the ax-eq-ax sequence. However, in the triaxial borate complexes (3) the bonds are tetrahedral if one were formed at the ax-eq-ax oxygen atoms, the bonds would be considerably distorted. However, the angles have much less effect on complex formation with cations which does not involve covalent bonds. 

Until recently, very little had been reported on the important area of metal borate complexation in aqueous solution. The effect of salts on the ionization of boric acid (358, 375) has been mentioned above, and subsequent research suggests that complexation of borate with, for example, calcium ions can account for the enhanced acidity of H O Literature on cationic complexes of boron was reviewed in 1970 (376). 

In the initial description of the cationic dye-borate system [24, 76], it was postulated that electron transfer was possible because, in nonpolar solvents, dye/borate salts exist predominantly as ion pairs. Since the lifetime of the cyanine singlet excited state is quite short [24, 25], this prerequisite is crucial for eflfective photo-induced electron transfer. Recently initiator systems in which neutral dyes are paired with triarylalkylborate anions have appeared in the literature [77]. In the latter case, the borate ion acts as the electron donor while neutral merocyanine, coumarin, xanthene, and thioxanthene dyes act as the electron acceptors. It is obvious that these initiating systems are not organized for effective electron transfer processes. The formation of an encounter complex (EC) between excited dye and electron donor is required. 

Through comparative studies for a range of [Tp M(CO)3] analogues with different substituents in the pyrazolyl ring, Lalor demonstrated that the oxidation by arenediazonium cations occurred in response to the steric rather than the electronic effect of the 3-methyl substituents. However, further steric crowding in either the hydrotris(pyrazolyl)borate ligand or the diazonium cation promoted a reversion to the carbonyl-substitution pathway, producing aryldiazenido complexes Tp M(NNAr)(CO)2, which are also the products observed for the... 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

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