Transition-metal derivatives bonding

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

The discussion of the main group 3-5 and 3-6 compounds in the previous sections was limited to examples in which the group 3 element E is three-coordinate, so that an empty p-orbital on E is available for overlap with a lone pair on the group 5 or 6 atom. For the same reason, the discussion here will focus on those compounds with three-coordination at gallium, indium, or thallium. In the case of the transition metal derivatives, it is transition metal -electrons that are available to overlap with the empty p-orbital on E to form the potential ir-bond, as illustrated in Fig. 26. 

Acceptor-substituted carbene complexes are highly reactive intermediates, capable of transforming organic compounds in many different ways. Typical reactions include insertion into o-bonds, cyclopropanation, and ylide formation. Generally, acceptor-substituted carbene complexes are not isolated and used in stoichiometric amounts, but generated in situ from a carbene precursor and transition metal derivative. Usually only catalytic quantities of a transition metal complex are required for complete conversion of a carbene precursor via an intermediate carbene complex into the final product. 

Cyclopropanes are cleaved by a variety of transition metal derivatives. The first observation was that PtCl4 reacts to give a platinocyclobutane. Cyclobutanes appear to be less reactive, but cubane reacts with [Rh(CO)2Cl]2 leading to C—C bond cleavage. The subject of C—C bond cleavage has been reviewed. 

Insertion of unsaturated molecules into a transition metal-silyl bond has been suggested for the catalytic reactions related to hydrosilylation and silylcarbonylation. However, there is little direct evidence supporting such a process for unsaturated molecules to insert into a metal-silyl bond in organometallic complexes. " Thus, the fact that 108 is readily derived from 11 and 13 demonstrates the participation of this process in the catalytic cycle of silylformylation. 

The thermodynamics of the oxidative addition process tends to be favored by increased electron density at the metal centre, hence the focus on later transition metal derivatives. Furthermore, as discussed above, it is believed that M—N n-bonds to the later transition metals are of significance only if the transition metal complex is unsaturated. Saturated late transition metal amides (parent or substimted) often exhibit the so-called n-conflict (see above) so that the nitrogen centre displays no n-bonding to the metal and retains its lone pair character and basicity. 

There is evidence in the literature that many of these transition metal derivatives, when chemically bonded or grafted onto an inorganic oxide surface, possess catalytic activity for olefin reactions far greater than that observed when the organometallic compound is used in a homogeneous manner (26). In some examples, catalytic activity was promoted when the original compound had none (3, 27). 

In principle the insertion of C02 into a transition metal hydrogen bond can result in either M-0 or M-C bond formation, i.e., production of metalloformate (4) or metallocarboxylic acid (5) derivatives. Thus far,... 

The increased reactivity of triflate groups bonded to silicon relative to halogen substituents can also be utilized for the synthesis of multifunctional cyclopolysilane transition metal derivatives, as has recently been shown for 1,3-disubstituted cyclohexasilanes104 (equation 35). 

A wide variety of compounds are known which contain at least one transition metal-tin bond. These derivatives undergo different types of reactions, such as substitution of ligands at the tin or the metal center, photochemical reactions and so on. Selected tin derivatives of such transition metal complexes are shown in Table 11. 

Cationic 1,2-silyl migration was proposed to be involved during the reactions of carbon monooxide and isocyanides with transition metal-carbon bonds (ligand distribution) (equation 18)55-65. Typically, the reaction of l-sila-3-zirconacyclobutane 29 with carbon monooxide afforded a dioxasilazirconacyclohexane derivative 30. The reaction was considered to proceed via a CO insertion into a Zr—C bond followed by a 1,2-silyl migration as shown in equation 1960. This type of reactions are well-documented in a review of Durfee and Roth well66. 

The carbon-iodine bond in perfluoroalkyl iodides is usually susceptible to homolytic fission this was exploited in early work on the synthesis of mercurials and in later work relating to group IVB and transition-metal derivatives (Figure 10.2). 

Factors affecting the stability of transition-metal bonds to carbon are of continued interest and fluorocarbon transition-metal derivatives are especially interesting [115-117] because of their generally enhanced stability, relative to hydrocarbon analogues. Factors... 

For a wider discussion of fluorocarbon-transition-metal derivatives, and aspects such as their bonding, the reader is referred to other sources [115-117]. 

Although there are two metals in the Ziegler-Natta catalyst, the weight of current evidence indicates that polymerization takes place at the transition metal-carbon bond. The mechanism is illustrated here with reference to polymerization by TiCl /Al(CH2CH3)3 catalyst complex. The normal geometry for Ti atoms is octahedral, and the catalyst site, as shown in 9-21, is believed to be a coordinately unsaturated Ti bonded to four Cl s [which in turn are bridged to two other Ti s] and to an alkyl group, derived from the aluminum alkyl component. 

Another possible explanation for enhanced stabilities of perfluoro-alkyltransition metal compounds concerns possible multiple bonding between the metal and the fluorocarbon group 230). Whatever the reason, a large number of fluorocarbon derivatives of the transition metals have now been described, and a study of their synthesis, properties, and reactions constitutes an important new branch of organotransition metal chemistry. Fluoroalkyltransition metal compounds were first described in 1959 J 8), while perfluoroary 1-transition metal derivatives were discovered in 1963 201, 234). Some typical syntheses of fluorocarbon-transition, metal compounds are shown below ... 

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