Tetrahedral complexes Equilibria with octahedral complexe

A conceptual question is whether simple octahedral or even tetrahedral complexes that have a dynamic Lewis-basic pendant group, donating a pair of electrons to the metal center, are suitable for the production of an elastomeric polypropylene. As shown in Scheme 7, a dynamic equilibrium may take place between a tetrahedral and an octahedral configuration (X=halide, E=donor group with a lone electron pair, R=C, N, P, or other anionic bridging group). (A plausible frans-octahedral complex, which can be formed in this type of dynamic process, is unable to perform the olefin insertion and has no catalytic activity [5,20,80,81].)... 

The polymerization of propylene using complex 14 activated by MAO (Al Zr ratio=500, solvent toluene, 25 °C) yielded 80 g polymer-mol Zrl-hrl with a molecular weight Mw= 115,000 and polydispersity=2.4 [119]. The reaction was carried out in liquid propylene to avoid, as much as possible, the epimerization of the last inserted monomer unit and to allow rational design of the elastomeric polymer. The formation of elastomeric polypropylene is consistent with the proposed equilibrium between ds-octahedral cationic complexes with C2 symmetry inducing the formation of the isotactic domain, and tetrahedral complexes with C2v symmetry responsible for the formation of the atactic domain (Scheme 7). The narrow polydispersity of the polypropylene obtained supports the polymerization mechanism in which the single-site catalyst is responsible for the formation of the elastomeric polymer. 

Because of the small stability difference between octahedral and tetrahedral Co11 complexes, there are several cases in which the two types with the same ligand are both known and may be in equilibrium. An example is that of the thiocyanate complexes in methanol. There is always some [Co(H20)4]2+ in equilibrium with [Co(H20)6]2+ in aqueous solution. 

The author demonstrated that from an aqueous solution that is dilute in nickel(II) ions, but concentrated in NaSCN, the tetrahedral Ni(SCN)4 complex can be extracted with hexanone. This shows that the octahedral [Ni(SCN)4(H20)] complex, which forms in presence of thiocyanate ion in very high excess, must be in equilibrium with small concentrations of the tetrahedral species. 

The concentrations of the two complexes increase with increasing Co(II) concentration, but at fixed [Co(II)j the absorbance due to the tetrahedral complex decreases quite rapidly with increasing water content [3] (Fig. 7). There exists an equilibrium between the octahedral and tetrahedral Co(II) species, and the octahedral complex is favored at high water concentration. This behavior can be easily understood if we assume that some of the water molecules are used to solvate the polar headgroups of CTAB molecules [2-4]. 

It is well known that the relative stability of octahedral and tetrahedral complexes in solution depends on the properties of ligands in addition to being dependent on the central metal ion. For a discussion of the latter effect it is desirable to possess data concerning the complexes of different metals with the same ligands. With the object of obtaining equilibrium constants fulfilling this requirement an investigation of equilibria of the type ... 

Assuming that in a saturated solution of MCl py (M Ni(II) or Co(II)) the only complex species present are MCl py (tetrahedral) and MCl py (octahedral) the solubility data together with the activity coefficients, found in the above discussed way, are sufficient for a calculation of equilibrium constants of l. It was defined as... 

A variety of geometries have been established with Co(II). The interconversion of tetrahedral and octahedral species has been studied in nonaqueous solution (Sec. 7.2.4). The low spin, high spin equilibrium observed in a small number of cobalt(Il) complexes is rapidly attained (relaxation times < ns) (Sec. 7.3). The six-coordinated solvated cobalt(ll) species has been established in a number of solvents and kinetic parameters for solvent(S) exchange with Co(S)6 indicate an mechanism (Tables 4.1-4.4). The volumes of activation for Co " complexing with a variety of neutral ligands in aqueous solution are in the range h-4 to + 1 cm mol, reemphasizing an mechanism. 

Spectral evidence" indicates an equilibrium between tetrahedral and octahedral Co" in iViV-dimethylacetamide and the equilibrium constant for [Co (tet)]/[Co (oct)] is reported at various temperatures. The complexes of acetylhydrazine (A), [CoA3]X2 (X = Cl or Br) and [Co(NCS)2A2]H20 and the tri-N-deuterio-analogue[Co(NCS)2(Ad3)2]D20 have been isolated and examined by i.r. Cationic complexes of JV-acyl hydrazines have been isolated with ligands in their keto-form, RCO-NH-NH2 however, the ligands also react in their enol form, RC(OH) = NNH2, forming neutral complexes (R = Me, Pr", Pr , or Ph). ... 

Much work has been devoted to the halide complexation of these elements in non-aqueous media. Equilibrium and calorimetric measurements for the formation of the [MX ](n-2) (M = Zn or Cd X = Cl, Br, I or SCN n = 1-4) anions in dimethyl sulfoxide (DMSO) have shown that stability constants follow the same order, but are much larger than those found for aqueous solution zinc exhibits an enhanced hardness as an acceptor in DMSO as compared to cadmium. Calorimetric measurements indicate a change from octahedral to tetrahedral coordination with increasing halide concentrations.1002-1006... 

Four-coordinate d8 complexes can display a closely related electronic and geometric equilibrium between paramagnetic tetrahedral and diamagnetic planar isomers. Numerous examples are known in nickel(II) chemistry (80). In this case, as well as with the octahedral complexes described above, there is no change in the coordination number of the metal ion. 

U.v.-Visible Absorption Spectroscopy.—A review of currently available u.v.-visible spectrophotometers and accessories has been compiled by Tayler, Several sample cells have been reported allowing absorption spectra to be recorded under non-ambient sample conditions. A high-temperature cell, designed for a Cary model 15 spectrophotometer, has been employed in an investigation of the octahedral-tetrahedral equilibrium in aqueous solutions of cobalt(ii) compounds. A cell for a double-beam instrument (Beckman Acta M-VII) enabled studies of aqueous systems with temperatures up to 325 °C at maximum pressures of 12 MPa.Sample pressure and temperature variation was also possible in a study of volatile uranyl complexes in the gas phase using a home-built spectrophotometer. ... 

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