Bonded alkyl and aryl ligands

Alkyl and aryl derivatives can be made by reactions such as 23.62-23.70, the last being an example of an oxidative addition to a 16-electron complex. Choice of alkylating agent can affect the course of the reaction e.g. whereas LiMe is suitable in reaction 23.62, its use instead of ZnMc2 in reaction 23.64 would reduce the M0F5. 

WClfi -I- 3Al2Me6 WMee -I- 3Al2Me4Cl2 MoFg -I- 3ZnMe2 MoMe -I- 3ZnF2 

Va Fig. 24.18 (a) The structure of the anion in Zeise s salt, K[PtCl3(r -C2H4)]. The Pt(II) centre can be regarded as being 

Hexamethyltungsten (equation 24.71) was the first example of a discrete trigonal prismatic complex (Box 21.4). It is highly reactive in air and is potentially explosive. 

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Whereas complexes of unsubstituted and substituted cyclopentadienyl ligands represent the vast majority of all published compounds in organolanthanide chemistry, examples of isolated and fully characterized (including X-ray structural analyses) compounds containing only cr-bonded alkyl and aryl ligands are still fairly rare. The first structurally characterized homoleptic lanthanide alkyls became available through the use of bulky mono-, bis-, and tris(trimethyl-silyl)-substituted methyl ligands. Simple unsolvated alkyls of the rare earth elements have not yet been synthesized. 

Apart from the platinum and nickel compounds with cr-bonded alkyl or aryl ligands, Chatt s group reported between 1961 and 1966 the synthesis of an impressive series of related cobalt(II), iron(II), rhodium(III), iridium(III), rhe-nium(III) and rhenium(V) complexes [39-42]. The paramagnetic cobalt(II) and iron(II) derivatives MR2(PR 3)2 were stable when R was an aryl group with... 

Simple homoleptic a-bonded alkyls and aryls have been difficult to characterize. They are most readily achieved by using bulky ligands or completing the coordination sphere with neutral ligands or by forming ate complexes. 

From an organometallic point of view, the o-bonded alkyl and aryl complexes are good precursors in the synthesis of metal-metal bonded derivatives. These latter compounds have been studied in order to determine the potentials for oxidation or reduction, electron transfer rates, and electron transfer mechanisms of metalloporphyrins as a function of solvent, axial ligand coordination, and of the macrocycle. Recently, bimetallic compounds have attracted growing interest due to their potential applications as starting materials for synthesizing polymeric conductors Some aspects of the reactivity for this family of compounds have been studied and will be discussed in this review as well. 

Once again, the trans cr-bonding alkyl or aryl ligand exerts a powerful influence on the M-N-O bond angle. Studies on transient intermediates during photolysis of Ru-NO nitrosyls have revealed different modes of binding (and dissociation) of coordinated NO at the ruthenium centers. Metastable NO linkage isomers have been observed for MNO (M = Fe, Ru, Os) and for MNO ° complexes of Ni, as well as for FeNO iron nitrosyl porphyrins [208-212]. 

Tetranuclear copper complexes have tetrahedral, square-planar, or butterfly structures. The compound Cu4 (Pr 0)2PS2 4 has a distorted tetrahedral skeleton of the metal atoms with one sulfur atom symmetrically bonded to two copper atoms, while the other sulfur atom forms a bond with the third copper atom. Copper and silver form many clusters containing alkyl and aryl ligands (See Chapter 4). The complex Cu4(CH2SiMe3)4, like Cu4 N(SiMe3)2 4, has a square-planar structure with bridging alkyl ligands. In the alkyl compound, in contrast to the amide one, there are Cu —Cu bonds. 

The above-mentioned data show that decomposition of complexes containing a M —C bonds occurs readily if such complexes have free coordination sites. The instability of compounds increases strongly if ligands possess hydrogen atoms in the P position (P elimination takes place readily). The stability of complexes with alkyl and aryl ligands decreases usually in the following order ... 

Rare earth organometallics have also been prepared with cyclooctatetraenyl (Fig. 11) and arene ligands, so have the cr bonded alkyls and aryl compounds. In the latter case, bulky and/or chelating ligands are typically used in order to achieve steric saturation around the rare earth center and hence the stability of the compounds. 

In addition to forming either single M—C bonds or salt-like ionic compounds, elements such as Li, Be, Mg, Al, form compounds with bridging alkyl and aryl ligands. This class of compound, which is already familiar... 

Many Cu(I) compounds have polymeric structures with weak Cu—Cu bonds that are bridged by atoms or groups. These include Cu(I) carboxylates, alkyls and aryls, alkoxides and (CuXL) complexes (X = halide, L = ligand). In Cu(I) compounds Cu has a filled 3d shell, 3d , that does not participate in metal-metal bonding, so the extent of metal-metal bonding in these compounds is questionable. Calculations show that the metal-metal bonding is at best weak . These compounds arise from the same syntheses as would be used to prepare the monomer, and so they are not considered further here. 

Simple alkyl and aryl cr-bonded complexes are conveniently prepared by reaction of an alkylating reagent with a halocobalt(II) precursor. All-alkyl systems are rare, but the penta-methylcobaltate(II) anion is known.197 More typically, the coordination sphere of the metal contains additional co-ligands, particularly with P, S, or N donors. Some examples that reflect the style of reactions extant appear below. 

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