Transition metal bonding failure

It has been noted (Section II,B,1) that reactions between transition metal carbonyl anions and silicon halides often fail to produce species containing silicon-transition metal bonds, and that such failure has been ascribed to nucleophilic attack by carbonyl oxygen. It is therefore interesting that compounds containing Si—O—C—transition metal linkages have recently been isolated from such reactions [Eqs. (105) (R = Me, Ph) 183) and (106)... 

Finally the ESR spectrum of Nb(7r-allyl)4/alumina was unaffected by the addition of ethylene gas to the ESR sample tube. It is assumed that polyethylene is produced in this process since polymer can be isolated from larger scale reactions under similar conditions. The accepted mechanism for the ethylene growth reaction postulates a steady-state concentration of a a-bonded transition metal-hydrocarbon species which would be expected to modify the ESR spectrum of the supported complex. A possible explanation for the failure to detect a change in the ESR spectrum may be that only a small number of the niobium sites are active for polymerization. Although further experiments are needed to verify this proposition, it is consistent with IR data and radiochemical studies of similar catalyst systems (41, 42, 43). 

In earlier chapters, allusions were made to die effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites ( 3.7.1) patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations ( 6.8.4) and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations ( 8.3.2). 

The proposed catalytic cycle35 involves the formation of a six-membered cyclic carbenium ion intermediate D with n-bonded metal. The failure of rearrangement of allylic imidates with a methyl group at the olefinic 2-position is presumably due to the difficulty of forming a tertiary carbon-metal bond (Table 4. entry 5)59. The stereocontrol of the metal-catalyzed rearrangement is different from the thermal one because of er-bonding of palladium or mercury in the transition slate. 

Existence of cyclobutadiene as the simplest cyclic hydrocarbon with conjugated double bonds was predicted by Hiickel in the 1930s. Until recently, however, all attempts to prepare this compound ended in failure. In 1956 Longuet-Higgins deduced 166) that coordination of cyclobutadiene with a transition metal should stabilize its unstable triplet state. 

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... 

The cleanliness of the supported copper and silver samples of Kavtaradze and Sokolova 200) has also been questioned 192). The suggestion of these authors 104) that the failure to observe stretching frequencies below 2000 cm" for CO adsorbed on the group Ib elements is a consequence of the electronic environment of the adsorbed species, and need not signify any structural bonding differences between adsorbed CO on these metals and on the transition metals is still a valid one. 

The initial indication that open-chain alkanes can also be activated by transition-metal ions came in 1969 when PtCl4 was found to catalyse the deuteration of alkanes in CH3COOD/D2O . This observation stimulated a search for complexes containing simultaneously M—R and M—H bonds, but initial failures to observe such compounds and the observation that C—M bonds in transition-metal complexes have low dissociation energies (ca 20-30 kcal mol" ) led to the belief that equilibria 15 and 16 were so strongly to the left that isolation of the oxidative addition products would be impossible . 

Attempts during the 1920s through 1940s to make further examples of d-block alkyls all failed. This was especially puzzling because by then almost every nontransition element had been shown to form stable alkyls. These failures led to the view that transition metal-carbon bonds were unusually weak for a long time after that, few serious attempts were made to look for them. In fact, we now know that such M—C bonds are reasonably strong (30-65 kcal/mol is typical). It is the existence of several decomposition pathways that makes many metal alkyls unstable. Kinetics, not thermodynamics. 

For many of the compounds discussed the failure of spectroscopic methods to indicate an unambiguous structure has made resort to X-r y analysis necessary. Structural studies have also been motivated by interest in the nature of metal-metal bonding. In this connection a recent paper on the general theory of bonding in cluster and ring compounds of main-group and transition metals is of interest. ... 

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