Structural studies carbon-metal bonds

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations.10 For example, ethylene (ethene, H2C=CH-2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3—C= (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif-... 

The covalent character present in many carbanion carbon-metal bonds means that we must use caution in discussing the properties of carbanions based on reactions of organometaUics. One way to study the structures of carbanions is to determine whether chiral carbanions undergo racemization. Studies of noncycHc carbanions indicate that the retention of configuration at a chiral carbanionic center depends on solvent and temperature, with solvents such as diethyl ether decreasing the covalent character of the carbon-metal interaction, and thus facilitating epimerization at the chiral center. 

Studies of the bonding of carbon monoxide to the metal surfaces produced structures in which the carbon atom is linked to one, two, or three metal atoms. The existence of bonds to two or three atoms (bridged bonds) has been questioned on the basis of theoretical calculations. None of these bondings, however, clarify the mechanism to any extent. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

The fulminate ion, CNO-, probably has a structure intermediate between C N O and C N 6 for since these two bond types have the same bond angles and term symbols ( 2), they can form intermediate structures lying anywhere between the two extremes. Which extreme is the more closely approached could be determined from a study of the bond angles in un-ionized fulminate molecules, such as AgCNO or ONCHgCNO, for the first structure would lead to an angle of 125° between the CNO axis and the metal-carbon bond, the second to an angle of 180°. 

Structural studies on transition metal complexes containing a-bonded carbon atoms Conformations of medium rings... 

The structures found for the products of the various reactions are often themselves of interest for example, the many cases where the olefin group becomes attached to the metal by a metal-carbon a-bond form a useful body of knowledge, and structural studies have proved an apparent case of the Wagner-Meerwein rearrangement in Pt(IV) and Au(III) complexes. 

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