Cyclopentadienyl transition metal complexes

Hydrocarbon ligands which formally contribute five electrons when bonding to a metal are called Dienyl ligands. Five-electron systems are found for cyclic 5-, 6- and 7-membered hydrocarbon ligands and non-cyclic ligands. The 7r-cyclopentadienyl group is the commonest ligand in this class and the chemistry of cyclopentadenyl metal complexes is discussed first. For convenience, the transition metal cyclopentadienide complexes are also discussed in this section. 

The cyclopentadienyl radical, CjHj, forms three distinct classes of complexes with transition metals. n Cyctopentadienyl complexes arc those which contain the cyclopentadienyl ring essentially covalently bonded to the metal, the metal being situated below the plane of the carbon atoms and usually equidistant from the five equivalent carbon atoms. In a few complexes the cyclopentadienyl radical may bind to the metal by only one carbon atom via a covalent 2-electron bond affording a-cyclopentadienyl complexes, which are discussed in Chapter 7,1. 4. 

The third class consists of the cyclopentadienides which behave chemically like the essentially ionic, alkali metal cyclopentadienides (see Vol. I). There is no abrupt transition between the ionic and covalent C5H5-metal bond. Indeed there is considerable ionic character in a number of complexes which are considered under the formal heading yr-cyclopenta-dienyl complexes . Even ferrocene, which is considerably covalent, has a charge separation between the metal atom and the tt-CsHs rings (see p. 102). Broadly speaking, the covalent character of the CsHs-metal bond increases steadily across the J-block transition series, i.e. with increasing atomic number, except in the case of manganocene which shows a sudden reversion to ionic character. This anomaly is discussed later. Transition metal cyclopentadienides are discussed on p. 110. 

Other ethers such as 1,2-dimethoxyethane are alternative solvents. The nature of the product may depend on the proportions of the reactants and in a number of cases a large excess of sodium cyclopentadienide gives both n- and o-cyclopentadienyl ligands in the same complex. Thus  

Further reactions may occur for example, the sodium cyclopentadienide may act as a reducing reagent  

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Table 1. Energy barriers to ring rotation in unsubstituted and substituted cyclopentadienyl transition metal complexes.

The compound Cr2(C5Mej)2S5 is the first chromium representative of sulfur rich dinuclear cyclopentadienyl transition metal complexes of general formula M2CP2S, (x 4). It is obtained from the reaction of Cr2(CsMes)2(CO)4 with sulfur in toluene solution. Alternatively, it can be prepared from Cr(C5Me5)2 and sulfur in about the same yield. 

D. The bonding in mono-n-cyclopentadienyl transition metal complexes... 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

Metallocene (Section 14 14) A transition metal complex that bears a cyclopentadienyl ligand Metalloenzyme (Section 27 20) An enzyme in which a metal ion at the active site contributes in a chemically significant way to the catalytic activity... 

Metallocene (Section 14.14) A transition metal complex that bears a cyclopentadienyl ligand. 

Transition Metal Complexes of Sterically Demanding Cyclopentadienyl Ligands... 

The scope of the present paper is limited to those cyclopentadienyl ligands that contain more than two bulky substituents and transition metal complexes derived thereof in order to be able to focus on the specific effects of these ligand systems. A selection of some mono-substituted cyclopentadienyl ligands will be treated also. Among the numerous reviews highlighting special aspects of cyclopentadienyl... 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

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