Perchlorates monodentate ligand

Perchlorate ion established as a monodentate ligand (to Co) by X-ray crystallography, following earlier spectrosopic and conductimetric indications of coordination (1961)... 

Even if the C104 ion is commonly believed to experience a scarce tendency to act as a ligand, nonetheless coordination of perchlorate as a monodentate ligand through an oxygen atom has been proposed in a number of nickel complexes containing alkylamines,1676 pyridine and substituted pyridines,1677-1680 arylamines such as aniline and substituted anilines,1681 phosphine and arsine oxides1682,1683 (Table 84). 

IR-spectral and conductivity data indicate that in chloroform solution diphenyl tellurium diperchlorate has monodentate, covalently bonded perchlorate groups. The complexes with one monodentatc ligand behave as 1 1 electrolytes1 in acetonitrile and acetone, while, the complexes with two monodentate ligands or one bidentate ligand behave as 1 2 electrolytes2. 

In the first compound the perchlorate ion is not coordinated to the nickel ion, but it acts as a monodentate ligand in the second and as a bidentate ligand in the third compound . 

Solid, hydrated nickel(II) salts and their aqueous solutions usually contain green [Ni(OH2)6l, the electronic absorption spectmm of which was shown in Fig. 20.21 with that of [Ni(NH3)g]. Salts of the latter are typically blue, giving violet solutimis. In aqueous solution, [Ni(NH3)g] is stable only in the presence of excess NH3 without which species such as [Ni(NH3)4(OH2)2] form. The violet chloride, bromide or perchlorate salts of [Ni(en)3] are obtained as racemates, the catimi being kinetically labile (see Section 26.2). The octahedral complexes trans-[Ni(C104-0,0 )2(NCMe)2] (21.52) and frans-[Ni(C104-< )2(py)4] illustrate the ability of perchlorate ions to act as bidentate or monodentate ligands respectively. The latter complex is discussed again later. 

Johansson et al. (1985) and Johansson and Wakita (1985) have studied the selenate and perchlorate systems using isomorphous substitution by yttrium to remove the ambiguities involved in the interpretation of the experimental data. The results indicate that selenate ions are monodentate ligands to R " while the perchlorate ions show no inner-sphere complex formation even at high concentrations. The coordination number was found in all systems to be invariably 8 0.3. This is the same value as Wertz and coworkers have assumed to be valid in concentrated chloride and bromide solutions. On the other hand, the neutron diffraction study by Narten and Hahn (1982) using isotopic substitution gave 8.5 0.2 for 2.85MNdCl3. 

The complex ion in octakis(pyridine-N-oxide)lanthanum(III) perchlorate (Al-Karaghouli and Wood, 1972a) is one of the very few examples of an eight-coordinate complex formed with monodentate ligands for which structural information is available. The complex ion is square antiprismatic and has almost perfect D4 symmetry. Consideration of the detailed coordination polyhedron suggests strongly that the ion is distorted toward the cube. 

Figure 17.23 F.xamples of monodentate. ehelaiing and bridging perchlorate ligands.

ICA 97)19]. Compound 163 (R = Br) and perchloric acid yield 164, where the monodentate pyrazolate ligand is protonated. Potassium hydroxide regenerates 163 (R = Br). Dicationic complexes of the type 164 can alternatively be produced from [Rh2(r/ -Cp )2(/x-0H)3]Cl04 and perchloric acid in the presence of excess pyrazole, 4-bromopyrazole, 3-methylpyrazole, or 3,5-dimethylpyrazole. 

The magnesium porphyrin radical, Mg(tetraphenylporphyrin)C104 (136), has been used as a model for the structural and stereochemical consequences of loss of an electron in photosynthetic chromophores.521 The primary photosynthetic reaction in plants and bacteria consists of a transfer of an electron, in the picosecond time domain, from the chlorophyll phototrap to nearby acceptors yielding chlorophyll -cation radicals. The structure of (136) shows a five-coordinated Mg2+ cation which is not quite symmetrically sited in the porphyrin ring but has metal-ligand distances similar to those found in the previous structures. The perchlorate anion is tightly bound (Mg—O = 2.01 A) in a monodentate mode. It was concluded that the porphyrin can act as an electron sink and that no major effects are found in the bond lengths, or on the stereochemistry, of the macrocycle. 

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