Reactivity octahedral complexes

The malonato complexes of chromium(III) are analogous to the oxalate complexes of chromium(III). Since malonic acid is a weaker acid than oxalic acid, the malonato complexes are expected to be more labile than the oxalato complexes. The dicarboxylate complexes of chromium(III) form a group of anionic complexes which are suitable for the study of octahedral complex reactivity. 

The Co system is more reactive as well as much more selective than the Ni and Rh catalyst systems (Table XVII). The best systems allow almost 100% conversion with almost 100% yield of c -l,4-hexadiene. The best of the Ni and Rh systems known so far are still far from such amazing selectivity. The tremendous difference between the Ni system and the Co or Fe system must be linked to the difference in the nature of the coordination structures of the complexes, i.e., hexacoordinated (octahedral complexes) in the case of Co and Fe and tetra- or penta-coordinated (square planar or square pyramidal) complexes in the case of Ni. The larger number of coordination sites allows the Co and Fe complex to utilize chelating phosphines which are more effective than monodentate phosphines for controlling the selectivity discussed here. These same ligands are poison for the Ni (and Rh) catalyst system, as shown earlier. 

In this section the in vitro reactivity of various octahedral complexes of technetium and rhenium are discussed and correlated with the in vivo pharmacokinetic data as observed for currently used radiopharmaceutical agents, which in most cases are indeed octahedral complexes of these metal centers. 

The chemistry takes place via an initial reduction of vitamin B12 or a similar cobalt (III) species 275, in a process that sees the conversion of cobalt from the +3 to the -1-1 oxidation state, and the opening of two sites of unsaturation, to afford 276 [74], This very reactive, highly nucleophilic intermediate reacts rapidly with the alkyl halide to form the octahedral complex 277, and reestablish... 

There have been extensive studies of the influence of an entering ligand on its rate of entry into a Pt(ll) complex.The rate constants for reaction of a large number and variety of ligands with trans-Pt(py)2C 2 have been measured (Table 4.13). The large range of reactivities is a feature of the associative mechanism and differentiates it from the behavior of octahedral complexes. The rate constants may be used to set up quantitative relationships. For a variety of reactions of Pt complexes in different solvents (Sec. 2.5.4) ... 

It was considered of interest to see how the reactions of benzyl isonitrile, a model ligand, are modified by its coordination to iron and to determine whether other ligands in the octahedral complex influence the reactivity of the benzyl isonitrile group. 

If we consider the geometry of the [M(en)3]"+ complex ion, we have further possibilities to consider. Whereas an octahedral complex with six identical ligands can only exist in one form, one with three didentate chelating ligands is chiral and can exist as two (non-superimposable) enantiomers (Fig. 2-7). The incorporation of polydentate ligands into a co-ordination compound may well lead to a rather considerable increase in the complexity of the system, with regard both to the stereochemical properties and any related chemical reactivity. 

The structure of titanium complexes affects the formation of hydrated titanium dioxide structure, since rutile and anatase lattices are composed of TiO octahedrons connected in definite manner. The formation of anatase structure occurs when two octahedral complexes form a common vertex. When two octahedrons are united via their edges, rutile structure is formed. Based on this assumption, it is considered that if titanium (IV) complexes with one reactive centre are formed during hydrolysis, anatase structure is formed if there are two reactive centres, then rutile structure is formed. 

Very limited reactivity studies have been reported for compounds of type (113). Complex (113 R = H, R = OMe) undergoes attack on electrophiles (E+X ) at oxygen to afford octahedral see Octahedral) complexes (114), with the electrophile counterion occupying the final coordination site. ... 

More important, the failure of many transition metal aqua ions to fit the correlations of Figure 8.4 highlights the influence of d electron configuration on the reactivity of metal aqua ions in substitution reactions. The importance of d electron configuration was first noted by Taube in 19521 and explained qualitatively in terms of valence bond theory. Taube, with his predilection for simple test tube demonstrations, distinguished labile metal complexes (ones which underwent substitution within the time of mixing) from inert ones, the latter being typically octahedral complexes... 

To gain better understanding of the assembly of the PDFlc-ec, a prominent member of octahedral complexes, studies were initiated to map the interactions in the Elec-E2ec binary complex. This is very important for a variety of reasons, not the least of which is to build a better model of the complex from rather low-resolution EM reconstruction studies. Also, the communication between the El and E2 components has an absolute dependence on ThDP reactivity. Earlier it was shown that residue F1407 on the inner loop of Elec participates in the reductive acetylation of E2ec. ° ... 

ReCl4 is built of a close-packed array of Cl atoms with pairs of adjacent octahedral interstices occupied by Re atoms the adjacent octahedra have a common face and the Re—Re distance is only 2.73 A, indicative of a bond.19 The substance is apparently only metastable and has rather complex reactivity which is not yet well understood. 

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