Lewis base ligands

A coordination compound, or complex, is formed when a Lewis base (ligand) is attached to a Lewis acid (acceptor) by means of a lone-pair of electrons. Where the ligand is composed of a number of atoms, the one which is directly attached to the acceptor is called the donor atom . This type of bonding has already been discussed (p. 198) and is exemplified by the addition compounds formed by the trihalides of the elements of Group 13 (p. 237) it is also the basis of much of the chemistry of the... 

Despite of the common reaction mechanism, peroxo complexes exhibit very different reactivities - as shown by the calculated activation energies -depending on the particular structure (nature of the metal center, peroxo or hydroperoxo functionalities, type and number of ligands). We proposed a model [72, 80] that is able to qualitatively rationalize differences in the epoxidation activities of a series of structurally similar TM peroxo compounds CH3Re(02)20-L with various Lewis base ligands L. In this model the calculated activation barriers of direct oxygen transfer from a peroxo group... 

In this reaction the metal behaves as a Lewis acid and accepts a pair of electrons from the Lewis base (ligand). In this case the ligand is water, with the oxygen atom donating one of its lone pairs to the nickel. The oxygen atom is called the donor atom. In this complex, there are six donor atoms. 

A Lewis base ligand has been developed as the chiral catalyst. Dahmen, Brase and coworkers reported that the [2.2]paracyclophane-based chiral N,O-ligand 8 (2 mol%) affords (V-formylamine with 84-95% (equation 34)134. In this system, the phenyl transfer reaction also proceeds in a catalytic manner134b. 

The second photochemical reaction which was studied was the reaction of CotCO NO with Lewis base ligands L (J 6 ). The observed solution phase photochemical reaction is carbonyl photosubstitution. This result initially did not appear to be related to the proposed excited state bending. Further reflection led to the idea that the bent molecule in the excited state is formally a 16 electron coordinatively unsaturated species which could readily undergo Lewis base ligand association. Thus, an associative mechanism would support the hypothesis. Detailed mechanistic studies were carried out. The quantum yield of the reaction is dependent on both the concentration of L and the type of L which was used, supporting an associative mechanism. Quantitative studies showed that plots of 1/ vs. 1/[L] Were linear supporting the mechanism where associative attack of L is followed by loss of either L or CO to produce the product. These studies support the hypothesis that the MNO bending causes a formal increase in the metal oxidation state. 

The [Cp (r 4-diene)MX] (M = Zr, Hf) complexes readily form four-coordinate adducts with neutral Lewis base ligands (e.g., THF or pyridine). Treatment of such stabilized adducts with Li R reagents leads to the formation of the corresponding alkyl complexes (e.g., 53). Some of these systems insert ethylene to form oligomeric chains55 (Scheme 17). 

Not all complexes are purely electrostatic. In fact, many metal complexes in biological systems have covalent interactions as well. In these cases, the ligand donates a pair of electrons (acting as a Lewis base) to the metal, which functions as a Lewis acid. Therefore, metals can be evaluated based on their abilities to accept electron pairs. Alkali metal ions (Na+, K+) and alkaline earth metals (Mg2+, Ca2+) tend to not form stable complexes with Lewis base ligands. Transition metal ions, particularly those with vacant ri-orbitals, will form more stable complexes with Lewis base-acting ligands. 

An investigation of the reaction of group 6 metal carbonyls with OH- in the absence of a Lewis base ligand indicates that oxygen exchange is a much more facile process than elimination of carbon dioxide from 53 (67). 

When CVD with copper(I) /3-diketonate complexes containing a Lewis base is carried out in the presence of hydrogen, deposition of metallic copper occurs by direct reduction with liberation of the Lewis-base ligand and formation of the corresponding -diketone, as was demonstrated for 10c (equation 6). In contrast, it is found that Cu(hfac)(l,3-butadiene) (lOg) deposits copper via disproportionation even in the presence of hydrogen . 

From the valence bond point of view, formation of a complex involves reaction between Lewis bases (ligands) and a Lewis acid (metal or metal ion) with the formation of coordinate covalent (or dative) bonds between them. The model utilizes hybridization of metal s, p, and d valence orbitals to account for the observed structures and magnetic properties of complexes. For example, complexes of Pd(ll) and Pt(Il) are usually four-coordinate, square planar, and diamagnetic, and this arrangement is often found for Ni(II) complexes as well. Inasmuch as the free ion in the ground state in each case is paramagnetic (d, F), the bonding picture has to... 

High selectivity (epoxide vs. diol) can be adjusted by temperature control, trapping of water, or the use of certain additives, such as aromatic Lewis-base ligands, which additionally accelerate the epoxidation reactions. Selectivities of > 95% can be reached. 

Valence bond theory pictures bonding in complex ions as arising from coordinate covalent bonding between Lewis bases (ligands) and Lewis acids (metal ions). Ligand lone pairs occupy hybridized metal-ion orbitals to form complex ions with characteristic shapes. 

The activity in MMA polymerization can be dramatically affected by the apical ligands. Apical aquo or alcohol ligands are labile and rapidly exchange with the polymerization medium. Lewis base ligands (e.g. pyridine, triphenyl phosphine) are comparatively stable. In MMA polymeri/ation, it is found that activity increases with the basicity of the ligand. With alkyl Co " complexes, a different order is found possibly because the type of apical ligand also controls the rate of initial generation of the active Co" complex. 

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