Metal-olefin complexes, bonding

In our laboratory we have examined the reactivity pattern of [0s3(y-H)2(C0)10], an unsaturated cluster which can be represented as possessing an osmium-osmium double bond in its classical valence bond representation. We find (2,3) that this compound undergoes a number of reactions with metal carbonyls which in some cases can be formulated as proceeding through intermediates analogous to metal olefin complexes ... 

This observation opens up a new possibility in the formation of C - O bonds for the already comphcated oxygenation reactions of organic substrates, i.e., the non-innocent behavior of the olefin in open-shell transition metal olefin complexes can allow a direct radical couphng of dioxygen with the coordinated olefin. 

Dr. Halpern I think the description can be formulated in a somewhat different way emphasizing the point that you raise namely, that formation of a metal olefin complex, by virtue of the back-bonding process, puts metal electrons into... 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

None of the theories proposed before 1951 to explain the nature of the bonding in metal-olefin complexes was entirely satisfactory (35). Chatt (S3) suggested that, in addition to the ordinary coordinate bond, some sort of bond involving the filled d-orbitals of the metal atom was essential for coordination of the olefin, but such a bond was difficult to formulate until Dewar (64) described it in terms of molecular orbitals. The structure which he proposed for the silver-olefin complexes, and that subsequently proposed for the platinum-olefin complexes by Chatt and Duncanson (35) are shown schematically in structures (I) and (II). The type bond, which has also been called a ji-bond (64, 4), is formed by the overlap of the filled bonding... 

The course of modern organometallic chemistry has been greatly influenced by three simple generalizations the Dewar-Chatt-Duncanson synergic bonding model for metal-olefin complexes (40, 72) Pauling s electroneutrality principle (174), and the 18-electron or inert gas rule (202). In this section the impact of recent theoretical calculations on these important generalizations will be evaluated. 

Chemical considerations suggest that metal-olefin back donation will be less important for silver(I) than for platinum(II), and Basch s ab initio calculations on [Ag(C2H4)]+ (75) have confirmed this view. These calculations suggest that most of the electronic rearrangement of the ethylene unit in this complex ion can be accounted for by the polarization effects induced by the positive charge on the silver atom. Indeed, the bonding metal-olefin molecular orbital has only 6.5% Ag 5s orbital character. This result agrees nicely with recent ESR studies on y-irradiated silver-olefin complexes which estimate a 5s spin density of 4.6% for this molecular orbital 92, 93). 

There have been too many crystallographic studies of transition metal-olefin complexes to present a comprehensive survey in this limited space. Therefore, only representative structures of major classes of compounds will be discussed, drawing on pertinent structural determinations as they are needed. Many of the important features of olefin bonding can be illustrated in the d10 system whore most of the complexes are approximately trigonal-planar (Fig. 4). 

The nature of the metal-olefin bond was studied recently in our laboratory by analyzing the natural bond orbital (NBO) results (Huang, Padin, and Yang, 1999b). The main feature of the bonding can be seen from the population changes in the vacant outer-shell s orbital of the metal and those in the d shells of the metal upon adsorption. The NBO analysis, summarized in Tables X and XI, is generally in line with the traditional picture of Dewar (1951), and Chatt and Duncanson (1953) for metal-olefin complex-ation, i.e., it is dominated by the donation and back-donation contributions, as illustrated by Fig. 13. 

In spite of the presence of usually bulky substituents at the silicon atoms, the Si—Si bond lengths are dramatically shortened and approach those of disilenes. This bond shortening is accompanied by a planar or almost planar arrangement of the substituents R1, R2 and the other silicon atom about each silicon atom. Such a geometrical arrangement resembles the bonding situation in transition metal-olefin complexes which, according to the model... 

The effective atomic number (EAN) rule is useful for interpreting how ligands with more than one double bond are attached to the metal. Essentially, each double bond that is coordinated to the metal functions as an electron pair donor. Among the most interesting olefin complexes are those that also contain CO as ligands. Metal olefin complexes are frequently prepared from metal carbonyls that undergo substitution reactions. 

For a flat surface, we would expect that both acetylene and ethylene would chemisorb initially such that the C-C bond vectors would be parallel to the surface plane. The hydrogen atoms could be significantly farther from the surface than for the benzene case discussed above. (They may he farther away as in ooordinately saturated metal olefin complexes where the acetylenic or ethylenic C-H bonds are bent from the C-C bond vector away from the metal center. However, generation of C-H-M multicenter interactions for the surface case may be substantial. The typically ooordinately saturated molecular olefin or alkyne complex may in this case he... 

The bonding in an M( rf-H2) unit is believed to have a strong formal resemblence to the /t-bonding of metal olefin complexes.79 In general, there is a combination of donation from the bonding orbital of H2 and back-donation from a suitable metal... 

The reactions of olefins and related unsaturated compounds play key roles in many of the reactions described in this book. Such reactions proceed via metal-olefin complexes, the bonding in which is related to that in the carbonyls here the forward donation is from the filled olefin C-C 71-orbital, while the back donation is from metal d-orbitals of the correct symmetry into the empty 71 -orbital of the olefin (Figure 2). The geometric constraints mean that ethylene for example lies perpendicular to the coordination plane of, and binds v - to, the metal. The classic example of this is found in Zeise s anion, Pt(C2Fl4)Cl3 . 

The synergic bonding in the metal-olefin complexes also reduces the bond order of the coordinated C-C bond, but these changes are less easily detected than those in the metal carbonyls. 

Figure 2 Diagrammatic representation of the synergic bonding in a metal olefin complex forward a-donation from a filled olefin n-orbital, balanced by back-donation from metal orbitals of appropriate symmetry into the olefin n orbitals.

The electrophilic activation of a C—C multiple bond as a result of coordination to an electron-deficient metal ion is fundamental to much of organometallic chemistry, both conceptually and in synthetic applications (11). The Wacker process, a classic example of an efficient catalytic oxidation, is an important industrial reaction, used for the conversion of ethylene into acetaldehyde. The catalytic reaction begins with the coordination of ethylene to a Pd(ll) center, leading to activation of the ethylene moiety. The key step is the reaction of the metal-olefin complex with a nucleophile to give substituted metal-alkyl species (12). The integration of this reaction into a productive catalytic cycle requires the eventual cleavage of the newly generated M—C bond. 

It may be assumed that Ni(0) first coordinates to the olefin before inserting into the a bond. Effectively, the rearrangement to butadiene has been shown to proceed through a metal-olefin complex . 

---