M-X bonds

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

Figure 9-1. Schematic plot of the (4s + 3dz2) and (4s - 3dz2) orbitals of a third-row transition-metal involved in a M-X bond.

The given conditions do not always allow for equivalent positions for all atoms. Take as an example the following conditions composition MX5, covalent M-X bonds, all X atoms bonded to M atoms. In this case all X atoms can only be equivalent if each set of five of them form a regular pentagon around an M atom (as for example in the XeFj ion). If this is not possible for some reason, then there must be at least two non-equivalent positions for the X atoms. According to the symmetry principle the number of these non-equivalent positions will be as small as possible. 

It should be noted that all the M=X bond lengths observed here are significantly shorter than those reported for the corresponding double bond compounds stabilized by intramolecular coordination of heteroatoms. In other words,... 

Remarkable progress in the chemistry of main group elements enabled us to synthesize and isolate heavy ketones which are capable of existence as stable species if their highly reactive M=X bond is adequately protected toward dimerization... 

Insertion of a ligand L into M-C or M-X bonds will of course also vacate a coordination site. 

The covalency contraction parameter, Rv, which measures the volume of a transition metal compound MmX relative to the volume of MgmXn, is proportional to the electronegativity of X and thus decreases as the covalence of the M—X bond increases. 

Because it is the rate of dissociation of the M-X bond that determines the rate of substitution, the rate law involves only the concentration of the starting complex, ML X. 

These experimentally observed structural trends were confirmed by computational calculations on H3A1 and Me3Al adduct families. In each adduct family, the amount of increase of the M-X bond lengths and decrease of the X M-X bond angles (X = H, Me) compared to uncomplexed MX3 diminishes with the atomic number of the group 15 element. The structural trends very well reflect the trends observed for the thermodynamic stability of such adducts, as is illustrated in Tables XI and XII. 

Thus, increased ionic character of the M—X bond should generally result in decreased d character (or increased s character) in the remaining apolar M—H bonds, which corresponds to increased d character (decreased s character) in the polar M—X bond itself. We have already seen for simple metal hydrides and alkyls this general correlation increased d character in hybrids is associated with more polarized bonds. 

In turn, each M—X bond engages in 3c/4e cu bonds with the opposite L donor of the other r 3-Cp ring. Thus, the following resonance structures summarize the primary bonding pattern, which can be compactly described as two tu bonds ... 

The general order of metal-carbonyl bond lengths is found to be Cr <bond lengths, except that W and Mo bond lengths are usually similar, with bonds at W being slightly longer (a feature... 

The type of reaction of [MX]+ ions will often depend on the strength of the M-X bond and a few examples of bond dissociation energies seem appropriate at this... 

In the following subsections, only cationic and anionic species containing at least one M-H or M-G bond are considered. Consequently, compounds containing exclusively M-X bonds (X = N, O, halide) are not discussed. (Interested readers are referred to Refs 178 and 179.)... 

When ER fragments are treated with transition metal halides, most often insertion of the ER fragment into one or more M-X bonds occurs (Equations (87)18 and (88)).18... 

The key observation is that there is a systematic relationship between the geometrical parameters defining the reaction . When the Y-M bond is lengthened (Ar) under the various influences of a particular crystal and molecular environment, the rest of the tetrahedral structure (YMX angle, M-X bond length, r,) responds in a standard, predictable, way. [See, for example, the correlation shown in Fig. 5, which is discussed also by Dunitz (1979).] This must represent, in the particular circumstances, the most favourable, i.e. the minimum energy, pathway for the structural change... 

This event leads to the formation of an electron donor-acceptor (EDA) complex involving the formation of a coordinate link between D and M. The availability of the additional electron pair at M causes an increase in electron density at X due to further polarization of the M-X bond. It is apparent that the amount of polarization will depend on both the polarizability of the covalent bond as well as the extent of interaction between D and M. For a given substrate the latter will depend on the donor properties of the donor8). 

The operation of the cis effect of the porphyrin may be explained according to Case D (Fig. 1) the more basic the porphinato ligand, the more electron density it induces at the metal ion, and the more will the d -electrons at the metal tend to flow away to the 7r-acceptor ligand X, reinforcing the M—X bond (90). An alternative ra-... 

Jack Halpern Perhaps I have more reason than anyone else to be disposed to the view that 7r-complexing is an important step of the insertion reaction, because I think that possibly we have the only reasonably clear cut case of an olefin insertion reaction where a complex is clearly implicated. This is the ruthenium chloride-catalyzed hydrogenation of certain olefins, which almost certainly involves the insertion of the olefin into a ruthenium hydrogen bond and where certainly a ruthenium olefin complex is involved as an observable reactant. Nevertheless, I am not at all sure to what extent this is a general or necessary feature of such insertion reactions. The important question is whether one or two coordination positions on the metal ion are involved in the transition state of the insertion reaction. For example, if one considers the insertion of an olefin, say into an M—X bond, then the transition state may look something like ... 

Hughbanks has calculated the molecular orbitals of a hypothetical 25-e [Mo6S8L5] cluster and discussed their dimerization (72). The 2eg and 2tIu o--acceptor orbitals lie above the manifold of 12 metal-metal bonding orbitals, which include some s and p hybridization acting to accentuate their projection outward from the cluster. These orbitals are strongly destabilized when the cluster is capped by donor ligands to form the dative exo M—X bonds. There remains one 2e (z2) acceptor... 

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