Transition metal alkyls and aryls

The Organamendlic Chemistry of the Transition Metals, Fourth Edition, by Robert H. Crabtree Copyright 2005 John Wiley Sons, Inc. 

METAL ALKYLS. ARYLS, AND HYDRIDES AND RELATED ff-BONDED LIGANDS 

The inherent stability of the R fragment plays a role. too. As an ion, CH3 is the most reactive. As we move to sp C Hs and even more to sp RCsC , where the lone pair of the anion is stabilized by being in an orbital with mote s character, the intrinsic reactivity falls off. The same trend makes the acidity of the hydrocarbons increase as we go from CH (pXa = 50) to C Hg (pK = 43) and to RC=CH (pX, = 25), so the anion from die latter is the most stable and least reactive. 

FIGURE 3.1 Schematic diagram showing qualitatively how the nucleophilic reactivity of main-group and transition metal alkyls to protoas or air oxidation depends on the alkyl itself and the electronegativity of the metal. 

The major decomposition pathway for alkyls is elimination (Eq. 3.1), which converts a metal alkyl into a hydridometal alkene complex. We study it in detail in Section 7.4. For the moment we need only note that this very common mechanistic type can occur whenever 

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The stirring discovery, that transition metal alkyls and aryls were accessible via oxidative addition of C—H bonds to electrophilic metal centers, soon brought up the question whether dialkyl, diaryl or alkyl(aryl) transition metal... 

The main classes of transition metal alkyls and aryls are the following ... 

From the evidence cited above it has been assumed that the formation of the O-sulfinates is a common feature of the insertion reactions of coordinatively saturated transition metal alkyls and aryls. The kinetic studies discussed in Section III refer to the scission of the M—R bond which yields the appropriate metal sulfinate. 

Compared in this section are the activation parameters and the relative reactivities toward SO2 of the various types of transition metal alkyl and aryl complexes. As shown in Table IV, the values of AS and AH range from — 63 to — 43 e.u. and from 2.7 to 8.7 kcal/mole, respectively. These entropies of activation are more negative than those for the solvent-assisted carbon monoxide insertion [—33 to —17 e.u. (1J7)] or for the SO2 insertion into the Sn—R (R = Ph and CHaPh)... 

Insertion of sulfur dioxide (SO2) into the metal-carbon bond of transition metal alkyl and aryl complexes has also been studied extensively. SO2 shows several binding modes to transition metals as shown in Scheme 7.15 because it is amphoteric, behaving as a Lewis acid and a Lewis base. The Lewis base character of SO2 provides the structural types r/ -planar (3) or (S,0) (4) where SO2 donates a pair of electrons to the metal accompanied by rr back-bonding from filled d orbitals of the metal atom. The Lewis acid behavior of SO2 as a ligand affords an 17 -pyramidal bonding mode (5) where SO2 accepts a pair of electrons from the metal. As ligands tike olefins or carbon dioxide generally tend to prefer... 

Subsequent studies on the reactivities of neutral and cationic alkyl- and aryl- palladium complexes revealed that the creation of a vacant site adjacent to the alkyl or aryl ligand causes marked enhancement in reactivity toward j8-hydrogen migration. The implications of these results on the fundamental processes of the transition metal alkyls and aryls with the mechanisms of Pd-catalyzed organic synthesis, such as arylation of olefins and carbonylation of aryl halides, have been discussed. 

On account of such rather discouraging results, there grew up a belief that transition metal alkyls and aryls were inherently unstable. It was suggested that transition metal—carbon cr-bonds must be very weak (thermodynamic instability). By implication an important mechanism for decomposition was thought to be homolysis to alkyl radicals, which led to mixtures of hydrocarbons (kinetic factors). 

R. J. Puddephatt, Chem. Met. Carbon Bond, 1982, 1, 245-285. Synthesis of Transition Metal Alkyl and Aryl Complexes. 

The organometaUic compounds of transition metals are fbrmd to be less stable than those of the main group elements. Since a large number of binary transition metal alkyls and aryls arise from coordinatively unsaturated... 

Another important property of transition metal alkyl and aryl bonds is their ability to undergo insertion reactions, e.g. 

I.r. and n.m.r. spectroscopy, electrical conductivities, and chemical evidence suggest that the insertion of sulphur dioxide into transition-metal-alkyl and -aryl bonds in complexes of the type [FeR(CO)2(A -C6H5)], [MoR(CO)3-(A -CgHg)], [MnR(CO)s], and [ReR(CO)s] involves the formation of intermediates which almost certainly have O-bonded sulphinate structures of the type (17) rather than S-bonded alkoxysulphenate structures of the type (18). 

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