Metal—ligand bonds insertion into

The interaction between catalyst and diazo compound may be initialized by electrophilic attack of the catalyst metal at the diazo carbon, with simultaneous or subsequent loss of N2, whereupon a metal-carbene complex (415) or the product of carbene insertion into a metal/ligand bond (416) or its ionic equivalent (417) are formed. This is outlined in a simplified manner in Scheme 43, which does not speculate on the kinetics of such a sequence, nor on the possible interconversion of 415 and 416/417 or the primarily formed Lewis acid — Lewis base adducts. 

Late transition metals have a marked affinity towards coordinating carbon monoxide and carbon-carbon multiple bonds. If there is another suitable ligand on the metal centre, this coordination might be followed by the insertion of a carbonyl group or the carbon-carbon moiety into the metal-ligand bond. Both types of attachment reactions are commonly exploited in catalytic processes and their characteristics will be discussed separately. 

In recent years there has been a growing interest in the chemistry of carbon dioxide, stimulated by current anxieties about alternative petrochemical feedstocks. One aspect under active exploration involves carbon dioxide activation via coordination to a transition metal, and indeed transition metal ions do form C02 complexes.177 The number of simple and reasonably stable complexes is still relatively small and usually limited to low oxidation state metal ions. There are many systems where C02 is used as a reagent giving rise to systems which, while not true C02 complexes, may simplistically be viewed as the products of insertion into metal-ligand bonds, e.g. reaction (9), where if L = H, formates are produced if L = OH, carbonates or bicarbonates result L = NR2 yields dialkylcarbamates and if L = R, carboxylate products result. Much of this area has recently been reviewed and will not be considered further.149... 

However with the chemical demonstration that diazenido and hy-drazido(2-) ligands can insert into metal-hydride bonds, the formation of 1H2H as a consequence of exchange 2H2 with a metal hydride at one of these intermediate stages remains a possibility. This would explain why the exchange is accelerated in the presence of dinitrogen. 

A conjugated diene can coordinate to a transition metal by only one double bond, as an s-trans-r 2 ligand, or with the two double bonds, as an s-cis-tf or as an s-trans-rf ligand [188]. A coordinated transoid monomer (as an s-trans-rj2 or an s-trans-rf ligand) is inserted into the metal-carbon bond, acquiring the syn-/ 3-allylic structure of the growing chain end. On the other hand, when a cisoid monomer coordinates to a metal (as an s-cis-rj4 ligand), an anti-t]2-allylic structure is formed. 

In addition to serving as ligands, organostannylenes can also insert into metal-ligand bonds (cf. Fig. 10). In the reaction shown in Eq. (50) (78), two moles of the divalent tin compound react, one forming a terminal stannylene complex, and the other inserting into the Pt—Cl bond. 

Many cases are known in which a ligand or molecular fragment appears to insert itself into a metal-ligand bond. Although some of these reactions are believed to occur by direct, single-step insertion, many insertion reactions are much more complicated and do not involve a direct insertion step at all. The most studied of these reactions are the carbonyl insertions these will be discussed following a brief introduction to some common insertion reactions. 

The formation of the thione complex 211 involves the coupling of two thioformaldehyde units, addition to the carbyne carbon of 210 to form a thiocarbene ligand, sulfur insertion into the metal-carbene bond, and... 

Transition metal catalyzed carbene insertion into the Si—H bond provides a direct and efficient method for the synthesis of organosilicon compounds. When chiral spiro diimine ligand (/ )-24a was applied in Cu-catalyzed asymmetric insertion of a-diazo-a-arylacetates with silanes, the Si—H insertion products were obtained in high yields (85-97%) and excellent enantioselectivities (90-99% ee) (Scheme 51) [26a],... 

Oxidative addition occurs when the metal inserts into the C—H bond to give new hydride and hydrocarbyl ligands (Scheme 11.8). The formation of two new metal-ligand bonds requires four electrons. Two electrons are derived from the C—H bond and two electrons are donated by the metal center. Thus, this process results in a formal two-electron oxidation of the metal center (Scheme 11.9). The C—H oxidative addition with a single metal center requires that the metal precursor have access to two vacant coordination sites and possess at least one lone pair of electrons. Such reactions... 

Insertion into transition metal-ligand bonds 6, 3.8.2.1.3... 

A quite spectacular insertion reaction of the earbene analogues 1 and 2 into transition metal ligand bonds is found with nickelocene [NiCp2] (Eq. 15). ... 

In this review we summarized the results of the latest ab initio studies of the elementary reaction such as oxidative addition, metathesis, and olefin insertion into metal-ligand bonds, as well as the multistep full catalytic cycles such as metal-catalyzed hydroboration, hydroformylation, and sila-staimation. In general, it has been demonstrated that quantum chemical calculations can provide very useful information concerning the reaction mechanism that is difficult to obtain from, and often complementary to, experiments. Such information includes the structures and energies of unstable intermediates and transition states, as well as prediction of effects of changing ligands and metals on the reaction rate and mechanism. 

In a formal sense, these are oxidation reactions, because rhodium(l) is oxidized to rhodium(ni). These are also addition reactions because two ligands are added to square planar sixteen-electron systems which are transformed into octahedral eighteen-electron complexes. These reactions can also be viewed as an insertion reaction (see (4) below) in which a metal atom is inserted into a bond between two nonmetals. 

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