Tetrahedral complexes Equilibria with square-planar complexe

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) 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. 

Acetylacetonates with bulky substitution on the 1 and 5 positions may form square planar complexes (22). Bis(2,2,6,6-tetramethyl-3,5-hep-tanedione)nickel(II) is square planar, whereas bis(2,6-dimethyl-3,5-hep-tanedione)nickel (II) exhibits an equilibrium between the square planar and tetrahedral configuration (22). 

An example of Eq. (a) occurs when L is the tetradentate, 1,4,8,11-tetraazaundecane , HjN(CHj)NH(CH2)3NH(CH2)2NH2. The laser pulse (1060 nm) produces an excess of the square-planar form in HjO, and reestablishment of the equilibrium occurs with a relaxation time of ca. 0.3 fis. An example of Eq. (b) is dichloro-l,3-bis(diphenyl-phosphino)propanenickel(ll), abbreviated [Ni(dpp)Cl2). Irradiation of this complex in CHjClj at 1060 nm produces an excess of planar form over the equilibrium concentration, whereas irradiation at 530 nm shifts the concentration to the tetrahedral form. The relaxation time in each case is 0.9 jas. Similar pulsed laser-rapid kinetics techniques are used to study the perturbation of equilibria involving the association-dissociation of Ni(II)-ligand complexes ... 

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]. 

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