Ligand sphere

Olefin Metathesis in the Ligand Sphere of Metal Complexes. 258... 

A strictly entropically controlled tendency for statistical ligand distribution was discussed 150) for ligand exchange when the sum of the Sb—X and Sb—Y bond energies remains constant. Calculations show that due to the electronic interaction in the entire molecule an energetic tendency also exists to form Lewis acids with mixed ligand spheres ... 

Fig. 4. Representation of the ligand sphere of the [2Fe-2S] cluster of the Rieske protein from spinach and the attribution of g-tensor to moleculEir axes as discussed in the text. Ser 130 has been observed to influence the redox potentiEd of the cluster via hydrogen interactions with the acid-labile bridging sulfur.

The classical view of the lone pair is that, after mixing of the s and p orbitals on the heavy metal cation, the lone pair occupies an inert orbital in the ligand sphere [6]. This pair of electrons is considered chemically inert but stereochemi-cally active [7]. However, this implies that the lone pair would always and in any (chemical) environment be stereochemically active, which is not the case. For example, TIF [8] adopts a structure, which can be considered as a NaCl type of structure which is distorted by a stereochemically active lone pair on thallium. In contrast TlCl [9] and TlBr [10] adopt the undistorted CsCl type of structure at ambient temperature, and at lower temperatures the (again undistorted) NaCl type of structure. The structure of PbO [11] is clearly characterized by the stereochemically active lone pair. In all the other 1 1 compounds of lead with... 

To account for stereochemical results for the epoxidation of allyl alcohols, a slightly different intermediate has been proposed as shown in Fig. 6.9.16 The authors propose an intermediate (A) analogous to the intermediate in peracid oxidations. A small molecule of alcohol or water is coordinated to Ti with deprotonation and another is coordinatively ligated to Ti without deprotonation to achieve a pentacoordinated ligand sphere. During epoxidation, the allyl alcohol substrate is held in position by a hydrogen bond. 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

The products of oxidation (alcohol, ketone, acid) lower the concentration of active complexes and, in addition, form complexes with a mixed ligand sphere with lower catalytic activity (kdi >kd2). The values of equilibrium constants Ain (Lmol-1) measured spectrophotometri-cally in a decane solution for cupric stearate + product are given below [70],... 

The authors proposed the one-step oxidation of alcohol in the ligand sphere of the Cu(II) (o-phen)2 complex [309]. 

To describe the d-orbital splitting effect for the octahedral field, one should imagine ligand spheres of electron density approaching along the x, y, and z axes, where the dxi yi and di lobes of electron density point. Figure 1.5 illustrates representations of high-probability electron orbit surfaces for the five d orbitals. 

In the following section, we give a survey of the known types of heteroboranes in question and discuss the structures in terms of the Wade-Williams rules. Readers, who are interested in the synthesis of heteroatom clusters, in their skeletal transformations, or in reactions at the ligand sphere, are referred to the cited literature. conjuncto- Boranes with heteroatoms in the skeleton are not considered in this brief discussion. 

SCHEME 9. Models for ligand spheres around a hthium cation... 

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