Equilibrium square-tetrahedral

The tetrahedral distortions in copper ICC of the type discussed, caused by R1 -substituents with different spatial effects, are strictly proved by x-ray diffraction, as well as confirmed by EPR studies [135,207]. However, a united point of view about the structure of these complexes in solution is still absent (compare Refs. 134, 135, 205 and 206). The existence of a square-pyramidal or distorted tetrahedral configuration, as well as the equilibrium square-tetrahedron, is accepted as possible. We emphasize that, in a series of cases, the nature of metal complex-former is the decisive factor, determining the structures of the examined ICC. Thus, practically independently of the character of the R1 -substituent, palladium chelates are planar, those of beryllium are exclusively tetrahedral, and those of cobalt, zinc, cadmium, and mercury are, in general, also tetrahedral [135]. 

Figure 3.8 An example of a nickel(II) complex that undergoes square-tetrahedral equilibrium.

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

Nickel(II) complexes of (505) exhibit spin equilibria in solution.1355 With the bidentate analogues (506), complexes [Ni(506)2] have been isolated.1356 When Rj = Ph, the complex is tetrahedral in solution. It has a temperature independent magnetic moment of 2.75pB- When R = Me, the complex exhibits square planar-tetrahedral equilibrium in solution. Both are, however, diamagnetic in the solid state. 

In the author s own laboratory the Cu(II)-catalyzed hydrolysis of the phosphate ester derived from 2-[4(5)-imidazolyl] phenol recently has been investigated146. The pertinent results are (a) the pre-equilibrium formation of a hydrolytically labile Cu(II)-substrate complex (1 1), (b) the occurrence of catalysis with the free-base form of the imidazolyl and phosphate moieties and (c) the extraordinary rate acceleration at pH 6 (104) relative to the uncatalyzed hydrolysis146. The latter recalls the unusual rate enhancement encountered above with five-membered cyclic phosphates and suggests a mechanism in which the metal ion, at the center of a square planar complex or a distorted tetrahedral complex, might induce strain in the P-O ester bonds (60). viz. 

Bis(pyrrole-2-aldiminato)nickel(II) complexes (125) are diamagnetic in the solid state when R = H, Pr, Pr and Et, and paramagnetic pseudotetrahedral when R = Bu. 1001-1003 In solution there exists an equilibrium between square planar and tetrahedral species when R = Pr1, Bus and Bu Such equilibria were also investigated for complexes of the type (126) obtained from the condensation reaction in basic media of o-aminobenzaldehyde and a number of diamines in the presence of nickel(II).1004-1007 Square planar complexes (127)1008,1009 and (128)1010 were obtained with deprotonated pyridinecarboxamide ligands. In these complexes the Ni—N (amide) bond distance (184-187 pm) is shorter than the Ni—N (pyridine) distance (192-195 pm). 

Nickel(ii) and cobalt(ii) complexes continue to be the most widely studied first-series transition metal complexes. The well resolved NMR spectra arise from the very rapid electron-spin relaxation which occurs as a result of modulation of the zero-field splitting of these ions. In the case of 4-coordinate nickel(ii), only tetrahedral complexes (ground state Ti) are of interest since the square-planar complexes are invariably diamagnetic. Many complexes, however, undergo a square-planar-tetrahedral dynamic equilibrium which can be studied by standard band-shape fitting methods (Section B.l). 

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