Stability of transition metal alkyls

Three years ago, while we were considering possible reasons for the general inability of transition metals to insert into and activate C-H bonds, our attention turned to the question of the instability of transition metal alkyl hydride complexes. We have listed the few alkyl hydride complexes of which we are aware (i) (one additional case (2) recently came to our attention) as well as some of the only slightly more numerous cases of substituted alkyl hydrides stabilized by chelation (3). In contrast, there are enormous numbers of polyalkyls (4, 5) and poly hydrides (6). While rarity does not logically imply instability, it does suggest it, so we considered possible mechanistic explanations for the assumed rapid decomposition of ci -MLn(R)(H) relative to cis-MLnR2 and cis-MLnH2. We have focused on octahedral complexes since they are both more important and more numerous. 

TT-Allylmethylnickel is typical of transition metal compounds in which the metal has an incompletely filled electron shell. It has at most 14 electrons in the valence shell. According to Chatt (7), the instability of transition metal alkyls is caused by the small energy difference between the highest occupied and the lowest unoccupied d orbitals. Thus electrons can be promoted easily to the next higher unoccupied orbital, leading to destabilization of the metal-to-carbon bonds and to decomposition of the compound. This concept may be used to explain the low stability of TT-allylmethylnickel since nonbonding d electrons or vacant d orbital must be present in this compound. 

A knowledge of such BDE s is important for an understanding of the kinetic and thermodynamic stabilities of organometallic compounds and of the thermodynamics of the many organometallic reactions that involve the formation or dissociation of transition metal-alkyl bonds, for example, insertion or oxidative addition-reductive elimination (Equations 2 and 3, respectively)(1-3). 

The stability of the metal—alkyl bond toward )S-hydrogen abstraction depends on the metal, its valency state and, very importantly, on the ligand environment. Conditions have been found which provide the transition metals of the left-hand end of the transition-metal series with a relatively high stability Ti,V, Cr, Mo can build good polymerization catalysts, and these have found industrial application for the production of polyethylene and copolymers. 

Stabilization of /j -alkyl and -aryl derivatives of transition metals can be enhanced by the judicious inclusion of various other stabilizing ligands in the complex, even though such ligands are known not to be an essential prerequisite. Particularly efficacious are potential IT acceptors (see below) such as AsPh3, PPh3,... 

Transition metal alkyls are often relatively unstable earlier views had attributed this either to an inherently weak M—C bond and/or to the ready homolysis of this bond to produce free radicals. Furthermore, the presence of stabilizing ir-acceptor ligands such as Cp , CO, or RjP was regarded as almost obligatory. However, (1) the M—C bond is not particularly weak compared say to the M—N bond, and (2) the presence of the new type of ligand on the metal could make the complex kinetically stable thus, even isoleptic complexes, i.e., compounds of the form MR , might be accessible 78, 239). These predictions have largely been borne out (see Table VII). 

The energies are usually expressed as electron volts. The IRE for the bond in ethane is zero and for CHgNa it is 2.56 ev. The stability of alkyl carbon-metal bonds for a variety of metals has been evaluated by Jaffe and Doak (5). They point out that not only is the (the measure of covalent energy) for the C—M bonds of transition metals appreciably smaller (perhaps one-half) than the corresponding values for other elements, but the ionic resonance energy of the alkyl-transition metal bonds is also appreciably smaller (perhaps one-third) than that of alkyl-alkali or alkyl-alkaline earth metal bonds. 

From the relative stabilities of the actinide homoalkyls or -allyls and the tris(cyclopentadienyl) actinide alkyls, it appears that a coordinatively saturated metal center is necessary for kinetic stability. In contrast to f-transition metal alkyls, the absence of hydrogens appears to be of minor importance. In the case of the lanthanide alkyls and the tetrabenzylthorium, where the formal coordination number is only four, the steric bulkiness of the Hgands must be responsible for their observed thermal stability. 

There would appear to be two distinct modes of reactivity of early transition metal alkyls with O2. When the metal is not in its highest oxidation state, an O2 complex of variable stability may form, and its subsequent reactivity may or may not involve the metal-carbon bond. The formation of remarkable stable 0x0 alkyls is an example of this pathway. In contrast, d°-alkyls react with O2 by a radical chain mechanism that invariable leads to formation of alkoxide complexes labile alkylperoxo ligands are clearly imphcated as intermediates in these reactions. 

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

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