Carbon monohapto

The carbonate ion, CO3-, by contrast, is a classic Werner ligand whieh forms innumerable complexes as a monohapto, dihapto or bridging donor. Examples of this latter mode... 

Phosphonium ylides, which can be written in the two familiar canonical forms, are available with a wide variety of substituents both at the phosphorus and at the carbon atoms (Scheme 30). In gold complexes, without any exceptions, they function as two-electron donors, as proposed by the dipolar form to give discrete Au-C cr-bonds (771, monohapto). No side-on, 7r-coordination (t 2), as might be expected out of the ylene form, has been observed to date. 

Zn-C bond of the monohapto ring is 2.223(4) A long, while the zinc-carbon bonds of the r -bound Cp ring span the wide range 1.991(4)—2.514(3) A. The short contact is the shortest Zn-G bond yet observed for a 7r-bound ring, while the long contact is barely a bond at all. The asymmetry of 22 is maintained in solution. 

Note that in a compound such as allyllithium, where a tr bond connects the carbon to the metal, the allyl group is referred to as monohapto or rjx. 

Hz). Several pieces of spectroscopic evidence lead to the conclusion the Me5C5 ring of V is not bonded in the monohapto manner. First, the 1h and 13c spectra of V indicate that the ring and Me carbons of the Me5C5 moiety are equivalent moreover, the equivalence of the methyl groups persists to -100°C and -80°C in lH and 13c NMR experiments, respectively. Second, the 31p chemical shift of V (111.0 ppm) is 33.8 ppm upfield (i.e. shielded) compared to that of the phosphorus(III) chloride precursor, 2Z 1° aH cases reported to date, phosphenium ion for-... 

These spectroscopic results are in agreement with the quantum mechanical calculations. CpaSi is used as a model for permethylsilicocene in order to reduce the number of internal coordinates It reacts as bis(monohapto)silicocene [5] with CO to a similiar adduct as found for the dimethylsilylene case The SiCO angle is calculated to be 159° and the silicon-carbon bond to be 210 pm, which underlines the reduced back-bonding in comparison to Me2SiCO The reaction enthalpy is, with -6 4 kcal/mole, nearly identical... 

The carbon-bound complexes show more diverse structural behavior than the side-bound species, for example, the Rh complex is strictly monohapto with respect to the metal as shown in II, while the Co complex has both oxygen... 

Finally, it may be mentioned that a number of stable /i1-C5H5—M or c-cyclopentadienyl—metal bonds are known and were, in fact, among the earliest stable a bonds from transition metals to carbon to be discovered. Their stability may be attributed both to their inability to decompose by /3-hydrogen elimination and to the coordinately saturated nature of the metal atoms in most of the compounds that contain them, monohapto-cyclopentadienylmetal compounds are noteworthy and are discussed further in the next Section, because they are usually fluxional. 

The mer-(CO)3(dppe)W(C=CHPh), 3, prepared as described below, contains a vinylidene ligand, a ligand that occupies a central role in the chemistry of monohapto carbon ligands in organometallic chemistry. Chemical transformations relate the vinylidene ligand to acetylides, -vinyls, acyls, and carbynes. Low oxidation state monohapto vinylidenes of tungsten have recently been implicated as catalysts in the polymerization of terminal acetylenes. ... 

This paper reviews the chemistry of metalloborane derivatives that contain borane or heteroatom borane groups which function as monohapto ligands. These compounds can be divided into three classes according to the number of electrons formally donated by the borane ligand to the metal. The electron pair acceptor class, represented by the compound Na[(OC)5Mn BHg], has received little attention thus far. The one-electron donor class, exemplified by the complex, l,2-(CH3)2-3- (C5H5)Fe(CO)2]-BioC2Ho, has a rich chemistry of metal-carbon and metal-boron derivatives. The third class includes two-electron donor derivatives that are represented by the compound (CHs)f,N[7,8-BoHioCHP - CrfCO),]. 

In this article the term organometallic compound includes alkyl and aryl derivatives of the rare earths—the transition metals of group III, scandium, yttrium, lanthanum and the lanthanides cerium to liitetium with covalent metal-to-carbon a-bonds, as well as the so-called 77-complexes with more than monohapto metal-to-carbon bonds, for example cyclopentadienyl and olefin complexes, metal acetylides, but not carbonyls, cyanides and isocyanide complexes. Derivatives of scandium, yttrium and lanthanum are included and discussed together with the compounds of the lanthanides, because of many similarities in the synthesis and the chemistry of these organometallic derivatives of the rare earths. 

Compounds with a (j-metal-carbon bond (monohapto M( yCC5H5). The IR spectrum is analogous to the cyclopentadiene molecule CsH and differs completely from IR spectra of the above-mentioned groups of compounds. 

Rapid migration of the metal around the ring in many ii -cyclopentadienyl complexes results in averaged carbon and proton signals for the ring atoms. If the barrier of this process is low, no line-broadening can be observed even at very low temperatures. In this case nmr spectroscopy cannot distinguish between pentahapto coordination and a fast equilibrium of monohapto-coordinated species. 

Representative work A Dedieu and F. Ingold, Angew. Chem., Int. Ed. Engl., 28, 1694 (1989). Monohapto Versus Dihapto Coordination of Carbon Dioxide in Bis(ammine) Nickel(O) Complexes. A CAS-SCF Investigation. C. Bo and A. Dedieu, Inorg. Chem., 28, 304 (1989). CO2 Interaction with HCr(CO)J Theoretical Study of the Thermodynamic Aspects. A. Dedieu and S. Nakamura, J. Organomet. Chem., 260, C63 (1984). lative Thermodynamic Stabilities of Acetyl and Formyl Complexes A Theoretical Determination. 

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