Weak ligand

Enone formation-aromatization has been used for the synthesis of 7-hydro-xyalkavinone (716)[456]. The isotlavone 717 was prepared by the elimina-tion[457]. The unsaturated 5-keto allyl esters 718 and 719, obtained in two steps from myreene. were subjected to enone formation. The reaction can be carried out even at room temperature using dinitriles such as adiponitrile (720) or 1,6-dicyanohexane as a solvent and a weak ligand to give the pseudo-ionone isomers 721 and 722 without giving an allylated product(458]. 

BeCl2 has an unusual chain structure (a) which can be cleaved by weak ligands such as Et20 to give 4-coordinate molecular complexes L2BeCl2 (b) stronger donors such as H2O or NH3 lead... 

The simplest iron-sulfur centers, which were first discovered in ru-bredoxins, consist of one iron ion coordinated by a distorted tetrahedron of cysteinyl sulfur atoms. This environment provides a weak ligand field giving a spin equal to and 2 when the ion is Fe(III) and Fe(II), respectively. It also determines the splitting of the ground spin manifold, and consequently the characteristics of the EPR spectrum. This splitting is generally described in the framework of the spin Hamiltonian ... 

At this point, we can schematically represent the structure of Cr(II) sites as (SiO)2Cr L , where L represents a weak ligand (oxygen of a SiOSi bridge) and n is a not fully known figure which increases upon activation at high temperature. The adsorption of CO at room temperature on grafted Cr(II) sites... 

Complexes containing the ylide ligand can be also obtained by reaction of the yellow solution of ylides (prepared by addition of butyllithium to tetrahydrofurane or diethylether solutions of the phosphonium salt) with [AulCfiFs) (tht)]. The weak ligand tht is replaced and the ylide complex is obtained (Equation 3.5) [70]. 

Sulfate process streams are commonly used in metal recovery because they are readily derived by leaching with sulfuric acid or by oxidation of sulfidic ores. Metal recovery from such streams rarely involves the formation of metal sulfate complexes because the sulfate ion is a weak ligand for base metal cations and consequently acidic ion exchange extractants are commonly employed (see Section 9.17.5), which generate sulfuric acid which can be returned to the leaching stage,... 

As a consequence of the weak ligand field of (L19)2-, the Mn11, Fe11, Co11, and Ni11 ions in the acetato-bridged complexes 61-63... 

One of the limitations of both 4 and 7 in catalysis is their ready decomposition to inactive cluster hydride complexes in the absence of substrate. If the substrate is a weak ligand (e.g., Me2C=CMe2), this decomposition can be competitive with cluster formation. A high concentration of substrate favors catalysis... 

RhAr6]+ ions are octahedral. As shown in Fig. 5, these observations are a manifestation of the Jahn-Teller effect in the gas phase. It must be stressed that other donor molecules easily displace these weak ligands this will be discussed in Section III.C.l. 

Carbon dioxide is another weak ligand that is able to coordinate to metal ions. The [FeC02]+ ion has been prepared by the reaction of Fe+ with p-butyrolactonc (135). 

There has been some interest in another weak ligand, N2 (9). The Fe+-N2 bond dissociation energy (136) has been determined to be 10.5 2 kcal mol-1 and a linear structure was suggested for the [CoN2]+ ion. Some later work has involved the calculation of the potential structures of other [MN2]+ ions. Thus the ground state... 

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