Octahedral complexes Olefins coordinated

We have thus far discussed rather generally the nature of restrictive ligand fields. We noted that these restrictions, even when small, could be sufficient to block molecular transformations which are, by themselves, thermodynamically only marginally favorable. Thus, simple olefins coordinated to metals that prefer square planar (with and lower metal complexes) and octahedral coordination should not be expected to undergo [2i-j-2s] cycloaddition. For molecular transformations which are energetically more favorable, the situation is different. Here, the energy provided by the exothermic [2 - -2s] process can reasonably be expected to compensate for attending restrictive field effects. We shall now consider the valence isomerization of quadricyclene (9) to norbomadiene (10). 

The activation energy of the insertion of coordinated ethylene estimated by the ab initio method was found to be 15 kcal/mol Despite the application of a more advanced calculation technique these results are less compatible with the experimental data on solid titanium chloride-based catalysts, when the activation energy of the propagation step is 3-6 kcal/mol (Table 10). Probably, this incompatability is due to the model used in ref. which describes the AC as a bimetallic complex CljTiCHj with A1(CH3)3. However, it is important to note that the calculations performed by means of the nonempirical method confirm the concept implying that in the active center the alkyl group occupies an intermediate position between the octahedral sites and that in olefin coordination the AC structure is reconstructed. 

Although the nature of seven-coordinate compounds is of intrinsic interest, perhaps of greater significance is the role of expanded coordination spheres as intermediates in the chemistry of octahedral complexes. The preparation of the thermodynamically unfavorable tmn5-Mo(CO)2-(diphos)2 isomer (5) via a seven-coordinate intermediate is an exemplary case of stereochemical control. Plausible seven-coordinate intermediates in the reactions of olefins with pentacarbonyltungstenphenylcarbene (6) can exert stereochemical control of the product distributions. 

Fig. 4. a) Coordination of an olefin (ethylene) to a transition metal (titanium), schematic representation of the relevant orbitals in the x—y plane of an octahedral complex (empty orbitals are shaded) ... 

As in olefin coordination compounds, acetylene complexes may have trigonal, square-planar, octahedral, trigonal-bipyramidal, and tetrahedral structures. In four-coordinate square-planar complexes, the acetylene molecule is perpendicular to the plane which comprises the remaining ligands and the central atom, while in trigonal and... 

The vacant sixth coordination site of these Ti centres can take up an olefin molecule to form the reaction complex required for the initiation and subsequent growth of polyolefin chains. Due to their octahedral dichelate-type structure, these Ti(III) centres are chiral and thus able to steer each incoming molecule into a preferred enantiofacial orientation. The stereospecificity with which subsequent propylene units insert into the growing polymer chain is most likely based on a mechanism analogous to that determined for soluble polymerization catalysts (Section 7.4.3). 

The effect of the last monomeric unit of the growing polymer chain on the stereospecificity of the olefin addition has been confirmed by the calculation of the energy of non-bonded interactions and by quantum-chemical calculations (see section 5.2). Corradini et al. have analyzed the possibility of the it-complex formation on the octahedral titanium ions located on different faces of a- andy-TiCla. The possibility of the coordination by both faces of the propylene molecule was studied. It was shown that active centers on the lateral faces of a-TiCls and y-TiClj may be regioselective (primary insertion of propylene) ruther than stereospecific (no predominant CjHs coordination by one face). In the case of active centers located on the edges of the layered modifications of TiClj, CsHg is coordinated with the more accessible (outward) coordination sites of the titanium ions predominantly the polymer chain is then located on the less accessible (inward) octahedral site. This position of the polymer chain results in a fixed orientation of the first carbon-carbon bond of the polymer chain due to its non-bonded interaction with the TiClj surface. This may explain the predominant coordination of propylene molecules by one face and the stereospecificity of such type of active centers. 

The next step is the oxidative addition of hydrogen, converting the square planar diastereomers of line 2 into the octahedral dihydrides of line 3 [93]. In the present system this reaction is the rate-determining step. The fast step following is the insertion of the coordinated olefin into one of the Rh-H bonds, giving rise to the two diastereomeric (T-alkyl complexes of line 4. By reductive elimination they generate the enantiomeric forms of the product, regenerating the catalytically active square planar species, which reenters the catalytic cycle. 

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