Olefin complexes stability

A series of five-coordinate, dicationic Rh(ll) and lr(ll) olefin complexes stabilized by N-donor ligands has been investigated in detail. Species of the type [lr°(iV4-ligand)(efliene)] + (162,179,180) [Rh (N3-ligand)(nbd)] + (181) (nbd = norbomadiene) and [M (iV3-ligand)(cod)] (182,183) have been obtained from their M(l) precmsors by one-electron oxidation using either [Fc] or Ag as an oxidant (Fig. 62). 

With the increase in electronegativity of the element M the degree of covalence of the bonds M —O and M—0 should increase, as a result of which an increase in electron density on the ion M can be expected. As in the formation of the ir-bond with olefin the ir-backbonding mechanism plays a large role, that should result in an increase in the ir-complex stability. 

The electronics behind the insertion reaction is generally explained in terms of a simple three-orbitals four-electrons scheme. Hoffmann and Lauher early recognized that this is an easy reaction for d° complexes, and the relevant role played by the olefin n orbital in determining the insertion barrier [26], According to them, the empty Jt orbital of the olefin can stabilize high energy occupied d orbitals of the metal in the olefin complex, but this stabilization is lost as the insertion reaction approaches the transition state. The net effect is an energy increase of the metal d orbitals involved in the d-7t back-donation to the olefin n orbital. Since for d° systems this back-donation does not occur, d° systems were predicted to be barrierless, whereas a substantial barrier was predicted for dn (n > 0) systems [26],... 

A quantitative treatment of tt complex formation is, however, more complicated, since it is generally recognized that all three wave functions are necessary for an accurate description of the bond. For instance, it has been pointed out by Orgel (27) that n complex stability cannot solely be the result of n electron donation into empty metal d orbitals, since d and ions (Cu+, Ag+, Ni , Rh+, Pt , Pd++) form some of the strongest complexes with poor bases such as ethylene, tt Complex stability would thus appear to involve the significant back-donation of metal d electrons into vacant antibonding orbitals of the olefin. Because of the additional complication of back-donation plus the uncertainty of metal surface orbitals, it is only possible to give a qualitative treatment of this interaction at the present time. 

We are currently trying to answer specifically the question of whether ir-bonded complexes do occur in certain cases where insertion reactions are observed. I think they do because I believe that the same factors which favor stabilization of this type of transition state will also tend to favor formation of 7r-bonded olefin complexes, which are only slightly removed from this. At the moment Bern Tinker is examining the insertion of olefins in mercuric complexes to see whether there is any indication of 7r-bonded intermediates. In his paper, Dr. Heck referred to some unpublished work relevant to this theme. I would certainly be interested in anything more he can tell us about that. 

Within the general description of ligand exchange, the relative stability of a variety of magnesium-olefin complexes/magnesacycles has been studied. For example, 1,4-diphenylbutadiene replaces the parent butadiene in equation 20 to form the penta-coordinated magnesium compound. ... 

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. 

The stability of the olefin complexes seems to be determined by the steric and electronic characters of both the phosphorus ligand and the olefin (22). For example, ethylene complexes have only been isolated for the cases with sterically large ligands such as P(0-o-tolyl)3 and PPh3 however, maleic anhydride forms a stable isolable complex with the smaller P(0-p-tolyl)3 ligand. The nickel-ethylene bond strength is estimated to be 39 kcal/mol based on values of 36 kcal/mol for 1-hexene and 42 kcal/mol for acrylonitrile [when L = P(0-o-tolyl)3] (22). 

It has generally been assumed that in olefin metathesis reactions the olefin first coordinates to the metal carbene complex, en route to the formation of the intermediate metallacyclobutane complex, and that after cleavage of this intermediate the newly formed double bond is temporarily coordinated to the metal centre. A number of stable metal-carbene-olefin complexes are known see elsewhere116,117 for earlier references. They are mostly stabilized by chelation of the olefin and/or by heteroatom substituents on the carbene, although some have been prepared which enjoy neither of these modes of stabilization118,119. 

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

Rh and Ir complexes stabilized by tertiary (chiral) phosphorus ligands are the most active and the most versatile catalysts. Although standard hydrogenations of olefins, ketones and reductive aminations are best performed using heterogeneous catalysts (see above), homogeneous catalysis becomes the method of choice once selectivity is called for. An example is the chemoselective hydrogenation of a,/ -unsaturated aldehydes which is a severe test for the selectivity of catalysts. 

The above results may reflect on the relative stabilities of the zwitteri-onic olefin complexes, with the least alkyl-substituted olefin, viz. (XIII), affording the most stable intermediate. Displacement of the olefinic C=C from the metal by the anionic terminus of the ligand furnishes the 5-sulfinate, possibly via its 0-bonded isomer. If this substitution occurs in a concerted fashion, then the product most likely will contain a rearranged allylic fragment [Eq. (22)]. An alternative mode of collapse... 

Olefin Complexes. Silver ion forms complexes with olefins and many aromatic compounds. As a general rule, the stability of olefin complexes decreases as alkji groups are substituted for the hydrogen bonded to the ethylene carbon atoms (19). 

The stability of a number of rhodium(I)-olefin complexes relative to that of the ethylene complex has been established by spectrophoto-metric determination of the extent of displacement by another olefin of C2H4 from (C2H4)2Rh(acac) in a closed system according to Eq. (4). 

These studies have indicated that (i) there is usually only one olefin molecule coordinated at each silver ion (621), (ii) alkyl substitution at the double bond decreases the stability of the complex (288, 416, 621), (in) with endo-cycloolefins, the stability constant increases with increasing ring strain (578), (iv) more stable complexes are formed with cis-than with diene complexes are formed by the 1,5-diene systems (416), and (vi) deuteration of an olefin increases the complex stability (154). 

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