Transition metal centres, oxidative addition

The photochemical formation of these complexes generally occurs from initial loss of CO or some similarly photolabile substituent from the transition metal centre. A common mode of attack of the Group 14 organometallic on the unsaturated species thus formed is by oxidative addition. There are many examples of such reactions, the most common involve E—H cleavage88 equations 40 and 41 show typical reactions. 

Some years ago, Dr. Cross and I put forward a description of concerted reductive elimination (and, by implication, concerted oxidative addition) processes at transition metal centres, assuming the conservation of orbital symmetry, within a single dominant configuration, for the most obvious reaction path This picture had unexpected implications which some recent work has rendered quite explicit, and which are discussed in Part II of this article. 

A convenient method for chiral carbene complexes is provided by oxidative addition of a C-Cl bond of a suitable precursor to a transition metal centre like palladium(O) [50]. The method has of course limited scope, since not aU transition metals have appropriate precursor complexes in low oxidation states. 

Interesting, optically acive complexes were obtained upon reaction of organosilicon hydrides. The reactions of R3SiH with transition metal complexes represent a key step in catalytic hydrosilylation reactions. Catalytic activation of silicon hydrides has been proposed to arise from an oxidative addition process to a transition metal centre. Such a process has been shown to be reversible via a reductive elimination step (equation 9). The stereochemistry of addition to an iridium complex was shown to occur in a cis fashion59 (equation 10). 

In marked contrast to the reductive elimination of all l, acyl and aryl-azolium salts, examples of mechanistically well-understood reductive eliminations of 2-haloazolium and azolium [i.e. C-H reductive elimination) salts from transition metal centres are much rarer, despite the ubiquity of transition metal-NHC complexes containing either coordinated halides or hydrides (or both). This may be due, in part, to the ease with which a 2-haloazolium or azolium salt can oxidatively add to a low valent transition metal centre, meaning that even if a 2-haloazolium or azolium salt were formed during a reaction via reductive elimination, a further oxidative addition reaction can rapidly re-form the starting complex. Furthermore, when an azolium salt is the decomposition product of a reaction, it is often not possible to discern whether this has been formed by a genuine NHC-hydride reductive elimination, or... 

The combination of hard (A) and soft (5) coordination in the 1,5-P2N4S2 ring system leads to a diversity of coordination modes in complexes with transition metals (Lig. 13.1). In some cases these complexes may be prepared by the reaction of the dianion [Ph4P2N4S2] with a metal halide complex, but these reactions frequently result in redox to regenerate 13.3 (L = S, R = Ph). A more versatile approach is the oxidative addition of the neutral ligand 13.3 (L = S) to the metal centre. 

There is an interesting paradox in transition-metal chemistry which we have mentioned earlier - namely, that low and high oxidation state complexes both tend towards a covalency in the metal-ligand bonding. Low oxidation state complexes are stabilized by r-acceptor ligands which remove electron density from the electron rich metal center. High oxidation state complexes are stabilized by r-donor ligands which donate additional electron density towards the electron deficient metal centre. 

There are three important routes to the formation of the mercury-transition metal bond (a) displacement of halogen or pseudohalogen from mercury(II) salts with carbonyl metallate anions (b) reaction of a halo-phenylmercury compound with a transition metal hydride and (c) oxidative addition of a mercury halide to neutral zero valent metals.1 We report here the syntheses of three compounds containing three-centre, two-electron, mercury-ruthenium bonds utilizing trinuclear cluster anions and mercury(II) halides.2-4... 

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. 

The design of transition metal complexes capable of C—F bond activation for the functionalization of fluorocarbons has attracted attention recently. It has been known for several years that oxidative addition of an aromatic C—F bond takes place at tungsten(O) to yield stable tungsten(II) metallacycles, the cleaved carbon and fluorine atoms both finishing up bound to the metal centre (Eqn. (2)) [34-36]. 

Note The apparent enhanced nucleophilicity of the metal centre in transition metal BIMCA complexes paired with facile oxidative addition on the metal should make this pincer ligand system a prime candidate in those catalytic reactions where the oxidative addition is thought to be the rate limiting step (Suzuki, Sonogashira, Heck). 

The transition metal activates the C-X bond in the oxidative addition step and normally the substrates have sp or sp carbons at or immediately adjacent to an electrophilic centre. The reactivity of aliphatic C-X bond towards the oxidative addition with a transition metal is somewhat low. However, in 1992, Suzuki and co-workers discovered that Pd(PPh3)4 can catalyze couplings of alkyl iodides with alkyl boranes at 60°C in moderate yields (50-71%). These conditions tolerated a wide variety of functional groups such as esters, ketals and cyanides. 

Accordingly, a broadening of MWD should be possible only by creating inhomogeneity in the active centre valences so as to promote different capabilities of monomer coordination and insertion and, thus, different propagation constants. The correspondence between narrow MWD and a unique oxidation state of the transition metal has been also pointed out by Christman for the ethylene polymerization with vanadium compounds-aluminum alkyls homogeneous systems. In this case, addition of a promoter causes re-oxidation of the deactivated sites (V") to the same identical initial ones (V "). 

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