Bonding in olefin-metal complexes

Some Notes on the Early Development of Models of Bonding in Olefin-Metal Complexes... 

Dewar s landmark contribution [32] did not receive much attention at the time it was published, possibly because the author did not seek to establish the experimental evidence for his model in subsequent publications. He seemed not to be very interested in the field of transition metal chemistry and was probably not aware that his description of the bonding in olefin silver complexes was supported by Raman studies reported a decade previously. In 1941, Harvey Taufen and coworkers had found that the olefin remained largely unchanged in its coordination to Ag+ and that the C=C bond was weakened only slightly by the formation of the olefin silver complex [36]. In contrast to Dewar, Joseph Chatt knew this paper and mentioned the results in a review on the mercuration of olefins, which like Dewar s article was also published in 1951 [37], In his paper, Chatt made a clear distinction between the olefin silver and olefin platinum complexes and argued that, in contrast to the ionized olefin silver(I) salts, in the olefin platinum(II) compounds the metal is present in a covalent state and not as an ion. He also believed that for Ag+ the d-shell was core like and not available in the manner necessary to stabilize the olefin-platinum bond [37]2. 

A major advance in theoretical aspects of organotransition metal chemistry came in 1951-1953 and has since become known as the Dewar-Chatt concept of bonding in olefin-metal compounds. Winstein and Lucas 254) had attempted earlier to explain the nature of the bonding in olefin-silver complexes on the basis of resonance stabilization ... 

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

The mechanism of this new reaction is shown in Scheme 14. Coordination of the diene to palladium(II) makes the diene double bond electrophilic enough to be attacked by the allylsilane. The attack by the allylsilane takes place on the face of the diene opposite to that of the palladium (anti). This is the first example of an anti attack by an allylsilane on a 7T-(olefin)metal complex. Benzoquinone (BQ)-induced anti attack by chloride ion produces the product 58. 

The reaction between alkenes and synthesis gas (syngas), an equimolar mixture of carbon monoxide and hydrogen, to form aldehydes was discovered in 1938 by Otto Roelen [1,2]. Originally called oxo-reaction , hydroformyla-tion is the term used today. This reflects the formal addition of formaldehyde to the olefinic double bond. Commercially, homogeneous metal complexes based on cobalt and rhodium are used as catalysts. With more than 10 million metric tons of oxo products per year, this reaction represents the most important use of homogeneous catalysis in the chemical industry. 

Laboratory in Oxford, and Geoffrey Ozin at the University of Toronto in the early 1970s. With the metal atom cocondensation technique (which as described in Chaps. 6 and 7 was also used to prepare a series of zerovalent arene and olefin metal complexes), they reported simultaneously that the elusive palladium and platinum tetracarbonyls, Pd(CO)4 and Pt(CO)4, as well as the coordinatively unsaturated fragments M(CO)3, M(CO)2, and M(CO) (M = Pd, Pt) were formed by cocondensation reactions of Pd and Pt atoms with CO in inert gas matrices at 4-10 K [119-122]. The comparison of the CO bond stretching force constants for Pd(CO)ra and Pt(CO)ra (n - 1-4) revealed that, in analogy to Ni(CO) , the most stable compounds were the tetracarbonyls. In a xenon matrix, Pd(CO)4 existed up to about 80 K [120]. Ozin s group as well as others... 

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.

Because replacement of 4-substituted pyridine-iV-oxide by the corresponding pyridine results in a higher frequency for vq c> Schmidt and Orchin suggest (3J4) that complexing is weakened by competition of the 7T orbitals of pyridine with the olefin tt orbital for the metal d-electrons. The coupling of the olefinic protons with platinum indicates also that the olefin is less strongly bonded in the pyridine complex (327). Fritz and Sellmann (232) contend, however, that, since does not... 

The mechanism of catalytic hydrosilylation involves oxidative addition of a silicon-hydrogen bond to a metal complex as an essential step since it is here the activation of hydrosilane by the catalyst takes place. Thus, many transition metal ions and complexes, especially group VIII metals in low oxidation state containing ir-acid ligands such as CO, tertiary phosphines or olefins display catalytic activity. The sequence of unit reactions in a typical d -metal complex-catalyzed hydrosilylation is summarized as ... 

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