Transition elements stereochemistry

This is the most common coordination number for complexes of transition elements. It can be seen by inspection that, for compounds of the type (Ma4b2), the three symmetrical structures (Fig. 19.6) can give rise to 3, 3 and 2 isomers respectively. Exactly the same is true for compounds of the type [Mayby]. In order to determine the stereochemistry of 6-coordinate complexes very many examples of such compounds were prepared, particularly with M = Cr and Co , and in no case was more than 2 isomers found. This, of course, was only negative evidence for the octahedral structure, though the... 

Tetrahedral complexes arc also common, being formed more readily with cobali(II) than with the cation of any other truly transitional element (i.e. excluding Zn ). This is consistent with the CFSEs of the two stereochemistries (Table 26.6). Quantitative comparisons between the values given for CFSE(oct) and CFSE(let) are not possible because of course tbc crystal field splittings, Ao and A, differ. Nor is the CFSE by any means the most important factor in determining the stability of a complex. Nevertheless, where other factors are comparable, it can have a decisive effect and it is apparent that no configuration is more favourable than d to the adoption of a tetrahedral as opposed to... 

Molybdenum and tungsten are similar chemically, although there are differences which it is difficult to explain. There is much less similarity in comparisons with chromium. In addition to the variety of oxidation states there is a wide range of stereochemistries, and the chemistry is amongst the most complex of the transition elements. 

ZIEGLER-NATTA POLYMERIZATION. Polymerization of vinyl monomers under mild conditions using aluminum alkyls and TiCL lor other transition element halide) catalyst to give a stereoregulated, or tactic, polymer. These polymers, in which the stereochemistry of the chain is not random have very useful physical properties. 

The VSEPR approach is largely restricted to Main Group species (as is Lewis theory). It can be applied to compounds of the transition elements where the nd subshell is either empty or filled, but a partly-filled nd subshell exerts an influence on stereochemistry which can often be interpreted satisfactorily by means of crystal field theory. Even in Main Group chemistry, VSEPR is by no means infallible. It remains, however, the simplest means of rationalising molecular shapes. In the absence of experimental data, it makes a reasonably reliable prediction of molecular geometry, an essential preliminary to a detailed description of bonding within a more elaborate, quantum-mechanical model such as valence bond or molecular orbital theory. 

The VSEPR model works at its best in rationalizing ground state stereochemistry but does not attempt to indicate a more precise electron distribution. The molecular orbital theory based on 3s and 3p orbitals only is also compatible with a relative weakening of the axial bonds. Use of a simple Hiickel MO model, which considers only CT orbitals in the valence shell and totally neglects explicit electron repulsions can be invoked to interpret the same experimental results. It was demonstrated that the electron-rich three-center bonding model could explain the trends observed in five-coordinate speciesVarious MO models of electronic structure have been proposed to predict the shapes and other properties of non-transition element... 

More recent developments in the mechanistic aspects of the alkene metathesis reaction include the observation that the alkene coordinates to the metal carbene complex prior to the formation of the metallacyclobutane complex. Thns a 2 - - 2 addition reaction of the alkene to the carbene is very unlikely, and a vacant coordination site appears to be necessary for catalytic activity. It has also been shown that the metal carbene complex can exist in different rotameric forms (equation 11) and that the two rotamers can have different reactivities toward alkenes. " The latter observation may explain why similar ROMP catalysts can produce polymers that have very different stereochemistries. Finally, the synthesis of a well-defined Ru carbene complex (equation 12) that is a good initiator for ROMP reactions suggests that carbenes are probably the active species in catalysts derived from the later transition elements. ... 

We shall set out here the structures of the compounds of Xe without enlarging on their stereochemistry. The close analogy with the structural chemistry of iodine will be evident, for the bond arrangements are for the most part consistent with the simple view of the stereochemistry of non-transition elements set out in Chapter 7. It will be equally evident that the difficulties encountered with certain valence groups in connection with the stereochemistries of Sb, Te, and I are also encountered here. 

Both Mo and W have a wide variety of stereochemistries in addition to the variety of oxidation states, and their chemistry is among the most complex of the transition elements. Uranium has sometimes been classed with Mo and W in Group VI, and indeed there are some valid, though often rather superficial, similarities the three elements form volatile hexafluorides, oxide halides and oxo anions which are similar in certain respects. There is little resemblancefirthe sulfur group except in regard fief stoichiometric similarities, for example, SeF6, WF6, SO , MoOj", and such comparisons are not profitable. 

Is substantial. By contrast, the fact that the valence AO s of A increase in energy in the order nd (n+l)s (n+l)p entitles these molecules to be formally "allowed" species, where the terms "forbidden" and "allowed" refer to the way in which the symmetries of the core and ligand MO s match. Since the nature of bonding within AX determines its stereochemistry, we can say that many differences between main group and transition element molecules which have yet to be discovered will be primarily due to the difference in core-ligand orbital matching as illustrated below, where the arrows indicate the way in which the orbital nodal planes increase ... 

Addition polymerization can be accomplished not only through a free radical initiator as mentioned above, but also by some other means. The most important polymerization catalyst is of the type known as Ziegler-Natta catalyst. These two chemists discovered that a combination of chemicals titanium tetrachloride and triethyl aluminum is an excellent catalyst for polymerizing a number of olefins. They were awarded Nobel Prize in 1963 for this discovery. Subsequent research by others found that similar combinations of chemicals a transition element compound and triethyl aluminum or similar alkylating agent do catalyze polymerization of olefins. Specific combination of such chemicals allow formation of polymers of specific stereochemistry. 

Any tentative explanation of the role of metal ions in enzyme action should consider the bond type, its stereochemistry, and its reactivity. Metal ions can act (i) as a link between enzyme and substrate, (ii) by changing the surface charge of the protein, or (iii) by both of these effects. Transition elements produce metal ions of variable valence. Their redox potential can be changed by complex formation. In this way catalysts of graded reactivity are realized in test tube systems and in living cells. The stereochemistry of the complex may contribute to enzyme specificity also. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

Predict the products of the following reactions on the basis of the reaction mechanism and anticipated transition structure. Be sure to consider all elements of stereochemistry. Unless otherwise specified, the reactants and reagents are racemic. 

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