Cyclooctatetraene complexes bonding

Table 2. Comparison of M—C bond lengths in several cyclooctatetraene complexes...

Another aspect of the chemistry of M(CO) fragments that the computed molecular orbital diagram could help to explain was the structure of the cyclooctatetraene complexes of iron and chromium tricarbonyl.10 Chromium tricarbonyl was shown to have three relatively low-lying vacant orbitals, with the right spatial characteristics to be able to accept electron donation from three of the double bonds of cyclooctatetraene, and accordingly adopts the r geometry shown in Figure 10.4. In iron... 

The cyclooctatetraene complexes, for example, [Nb(C8H8)3] and CH3Ta(C8H8)2, have rf, butadienelike, and rf-allyllike bonding of the rings to the metal they are nonrigid. 

Spectroscopic investigation (227) of the cyclooctatetraene complex (CgHg)PdCl2 indicates that all double bonds in the molecule are coordinated, but the structure of the complex has not been determined. When suspended in methanol, this complex is rapidly solvolyzed to produce (1, - dichlorobis(2 - methoxy - 3,5,7 - cyclooctatrienyl)dipalladium(II) (501). Similar complexes are formed by the reaction of 1,3-cyclohexa-diene, 1,3-cycloheptadiene, or 1,3- or 1,5-cyclooctadiene with Na2PdCl4 in methanol at room temperature. The last species reacts with HCl to give (l,5-cyclooctadiene)dichloropalladium(II) (501). Pyrolysis of the complex [(CgHi20CHa)PdCl]2 obtained from 1,5-cyclooctadiene yields 1-methoxy- and 2-methoxy-l,3-cyclooctadiene (535). 

Cuprous chloride forms crystalline complexes with numerous unsaturated hydrocarbons, of which one has already been mentioned. In several of these Cu(i) forms two bonds to Cl and a third to a multiple bond of the hydrocarbon, the three bonds from Cu being coplanar. In the 1,5-cyclooctatetraene complex, CuCl. the Cu and Cl atoms form an infinite chain (Fig. 25.5(a)), while in... 

The dication adopts a three-legged piano-stool configuration in which the O-U-O angles vary from 84.0° to 88.2° with a mean value of 87(3)°, and average COT-U-O angles of 127(1)°. The uranium atom is 1.92(2) A away from the planar cyclooctatetraene ring, and the mean U-G bond distance is 2.65(3) A. These values compare well with those determined in the monocationic cyclooctatetraene complexes [(COT)U(NEt2)(THF)3]+.161... 

The cycloheptatriene and cyclooctatetraene complexes show additional bands in the low energy region due to the presence of extra double bonds in the ring. 

It is also not clear why the diolefin 1,5-cyclooctadiene forms a weaker bond to silver than cyclooctene. The effects of strain differences within the olefins and/or within the complexes is not apparent30, but in any case suggests, as noted by the original authors, a negligible interaction between the metal and the second double bond. However, the cyclooctatetraene complex is asserted to be polymeric in solution and so currently precludes obtaining useful information derived from these complexation studies of this species. 

The silyl-substituted f/4-cycloheptatriene and -cyclooctatetraene complexes of Fe and U have been investigated to obtain PE spectroscopic information concerning substituent effects on either fluxional behavior or bonding their first ionization potentials are lowered by 0.2 eV247 or remain constant248 with respect to the parent systems. 

Crystallographic data, bonding distances and angles in rare earth cyclooctatetraene complexes. 

In the latter part of Table X, magnetic resonance data are listed for some chelating diolefins containing the ir-(C2)M bonding unit. These include the complexes of bicycloheptadiene (C7Hg) (V), cycloocta-1,5-diene (CgHi2) (VI), and a selected number of cyclooctatetraene complexes in... 

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

The crystal structure of (ij4-cyclooctatetraene)(hexamethylbenzene)ruthenium (16) indicates bonding as a tetrahapto ligand60. For this complex and similar iron-, ruthenium- and osmium-(ij4-cyclooctatetraene)(arene) complexes, their XH and 13C NMR spectra exhibit only a single signal for the cyclooctatetraene ligand at temperatures as low as —145 °C. Using this temperature, the barrier-to-metal migration is estimated to be <6.6 kcal mol 1. 

Tetrahaptocyclohexatriene- and cyclooctatetraene-metal-tricarbonyl complexes undergo some unusual cyclo-addition reactions with tetracy-anoethylene in which the dienophile undergoes a 1,3 addition across the coordinated ligand and causes a rearrangement of the metal-carbon bonds, for example (99),... 

Nomenclature system . The prefix 17 indicates that all unsaturated carbon atoms of the ligand indicated are ir-bonded to the metal atom for complexes in which the metal atom is bonded only to particular carbon atom(s) of a ligand, the total (x) of such metal-bonded atoms is indicated by a superscript (tj ) or, for ambiguous cases, by preceding locant designators, e.g. (tj -allyl), (1-4-Tj-cyclooctatetraene). 

Neutral ligands can also bond in a variety of ways. Cyclooctatetraene can act as an alkene (Tj2), a diene (T)4), a triene (T]6), or a tetraene (r 8), and the reactivity of the ligand changes accordingly. These are all ft complexes with the metal above or below the black portion of the ring and with the thick bond to the metal at right angles to the alkene plane. 

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