Olefin complexes structure

Synthesis, structure and reactions of chelate metal-olefin complexes... 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

In the case of r)2-coordination of the exocyclic C=C bond, it becomes substantially elongated compared with the double bond of free alkenes, as a result of back donation from the metal to the 7t orbitals of the double bond. For instance, in complex 17b the coordinated bond length is 1.437 A (see Fig. 3.2).18 This is also reflected in the loss of planarity around the quaternary exocyclic carbon, the methylenic carbon being bent out of the ring plane by 10.78°.18 Similar structural features were also observed with other P2Pd conjugated olefin complexes.39... 

In contrast, methyl cyclopropenone is reported283) to react with the Pt-olefin complex 455 at low temperature with replacement of the olefin ligand. In the resulting complex 456 the cyclopropenone interacts with the central atom via the C /C2 double bond according to spectroscopic evidence284). At elevated temperatures a metal insertion to the C1<2)/C3 bond occurs giving rise to 457. Pt complexes of a similiar type were obtained from dimethyl and diphenyl cyclopropenone on reaction with 455 and their structures were established by X-ray analysis285). 

Looking at C2-symmetric catalysts, there exist in principle two different metallocene-olefin complexes (Fig. 4). Structure a is energetically favored because the methyl group of the propene is in trans-position relative to the (3-methyl structure of the polymer chain. Both methyl groups in structure b are in cis position to each... 

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

Duncanson (i) of the molecular orbital bonding concepts of Dewar (2), which he developed to explain the structure of Ag+-olefin complexes, led to the suggestion that ethylene is symmetrically coordinated to the metal. Platinum, atomic number 78, has the electronic configuration of the xenon core (Is 2s 2p 35 3p 3d Z = 54), then... 

The proton-olefin complex is probably responsible for the unusually high cisjtrans ratio 47, 92). These intermediates have to be considered as hydrogen bond-like structures and evidence has been presented for an extremely high mobility of the proton in these structures 98, 99). 

Various authors have suggested that olefins and other unsaturated hydrocarbons may form n complexes by associating with a single atomic center of the catalyst. These structures are assumed to be analogous to the -IT olefin complexes (33,34) of the transition elements which have been the subject of recent intensive investigations (35). 

NQR, 22 216 olefin complexes of, 4 85 in organogermanium compounds, 27 141,143 oxyfluoride, properties of, 11 29 pentacarbonyl dimets of sulfur oxydifluoride imide, 19 203-205 pentafluoride, structure, 27 102 peroxides, 6 325-326 phthalocyanine, 7 54 physical properties of, 11 18 polysulfide complexes, 31 100, 102 envelope conformation, 31 115 synthesis, 31 103-104 [Pt[15]aneS5) f, 35 75, 77 reaction with fluorinated peroxides, 16 120 salts, lattice energy and thermochemistry, 22 52-56... 

The calculated transition state for -hydrogen transfer (Fig. 3) has a non-planar 6-membered ring structure. There is no direct interaction between the aluminium atom and the hydrogen being transferred. Thus, the situation differs sharply from that for transition metal polymerization, where the transition state has some resemblance to a hydride-bis(olefin) complex, as illustrated for a typical metallocene case in Fig. 3. 

Stmctural features of olefin cycloadducts (i.e., isoxazolines) were extended when regioselective 4-endo- and 3 -exo-deprotonations were discovered, with the broad range of ensuing reactions of these carbanions that are common with electrophiles (19-22, 26-28). Thus, sequences consisting of cycloaddition-modification-ring cleavage were utilized to build up complex structures (17, 20-23). 

For an X-ray crystal structure of a bissulfoximine-copper(I) olefin complex. 

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... 

Chelating polyolefins displace carbon monoxide from chromium hexa-carhonyl to form stable olefin complexes. Thus cyclo-octa-1,5-diene gives the yellow complex [Cr(CO)4(C8H12)] for which the cfs-structure (VI M = Cr) is proposed (79). 

Perfluoroethylene was first thought to react with iron carbonyl to give the iron(O) olefin complex [Fe(CO)3(C2F4)2] 213). It has since been shown that the product is a heterocyclic derivative of iron(II) (structure XII) 150, 214) and not a true olefin complex. 

The first class of complexes are often analogous to olefin complexes (see Section III). Thus the acetylene is intact in the complex, it can be recovered unchanged in many cases, and it is ir-bonded to the metal through its CjC bond, as shown by X-ray structural determinations and changes... 

Reduction of an alcoholic suspension of m-[PtCI2(PPh3)2] with hydrazine hydrate in the presence of an acetylene (ac) gives stable complexes of the type [Pt(PPh3)2(ac)] (41). The infrared spectra of the complexes indicate that the C C bond of the acetylene has been reduced almost to a C C bond, and preliminary X-ray studies (173) support the planar structure (XXXIX) in which the bonding is similar to that in the corresponding olefin complexes... 

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