Perchlorate Ion Coordination and Methods of Identification

X-ray crystal structure analysis has unequivocally established coordination of perchlorate in a number of perchlorate complexes. The O—Cl bond length of the coordinated part is generally greater than 

Hathaway and Underhill (15) were the earliest to suggest the use of IR and Raman spectroscopic methods to investigate perchlorate coordination to a metal ion. Perchlorate ion with its T symmetry should 

Monodentate attachment to a metal ion lowers the symmetry of perchlorate to C3v and bidentate attachment to C2v (15-17). Consequently the number of vibrational modes should increase (Table I). In addition, a metal-oxygen stretching frequency would also be expected in the far-IR region and has been located in the range 360-290 cm-1 (18). These effects resulting on coordination, particularly the increase in the number of vibrational modes, may be used for identifying coordination of perchlorate. 

It must, however, be borne in mind that minor shifts and weak splittings of the IR bands may arise on account of lowering of site symmetry because of strong lattice effects or of coupling of vibrations between perchlorate groups or from a purely isotopic effect within the group (15, 16). For example, the broad and strong band due to the v mode of ionic perchlorate is often split because of lattice effects. Despite these limitations, with a little care and caution, coordinated and noncoordinated perchlorate(s) are conveniently identified by IR spectroscopy. 

Perchlorate coordinated to a metal ion in the solid state generally is displaced in a polar solvent by a solvent (or another ligand) molecule. However, under favorable conditions, perchlorate coordination may persist in solution. In such cases, conductivity measurements are helpful in detecting the coordination of perchlorate (28). For instance, the conductance values of some lanthanide complexes Ln(hmpa)4(C104)3 (Ln = La, Nd, Gd, Dy, Yb) in nitrobenzene correspond to those of uniunivalent electrolytes, implying coordination of two of the perchlorates (29). The coordinated perchlorates are, however, easily displaced by the addition of hmpa as shown by the conductance values, which gradually increase to that corresponding to a 1 3 electrolyte. 

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