Replacement, ligand

Instead of electrostatic (or physical) adsorption, metal uptake onto oxides might be considered chemical in nature. In chemical mechanisms, the metal precursor is envisioned to react with the oxide surface, involving as surface-ligand exchange [13,14] in which OH groups from the surface replace ligands in the adsorbing metal complex. In this section it will be shown that a relatively simple electrostatic interpretation of the adsorption of a number of catalyst precursors is the most reasonable one for a number of noble metal/oxide systems. 

The mechanism sketched in Figure 3.18 implies that the starting complex L M has a free coordination site (or a readily replaceable ligand) and can act as an electrophile. Therefore reactions of this type will occur more readily with increasing nucleophilicity of the ylide and increasing electrophilicity of the metal complex... 

We are, however, focussing on using such a unit as a means of introducing possible analogs of intermediates in N2 fixation such as hydrazines, via modification of routes proven for such transformations in simple mononuclear Mo O complexes (81). In addition, synthetic routes to clusters containing more readily replaceable ligands on Mo (such as CO) are currently being explored. 

The kinetics of removal of iron(III) from its complexes with the aminocarboxylate-anthraquinone analytical reagent calcein and with the antitumor anthracycline doxorubicin by l,2-dimethyl-3-hydroxy-4-pyridinone (LI, (251) with R = R = Me) have been monitored. Rate constants for metal removal are almost independent of the concentration of the replacing ligand, indicating dissociative mechanisms they are approximately 1 x 10 s for displacement from doxorubin and between 12 x 10 s and 2 x 10 s from calcein. 

One further method to produce tris-chelates requires the use of a bis-chelate complex having two replaceable ligands. This will be considered later. 

Allyl)CpFe(CO)(PR3) complexes (14) have been prepared from the dicarbonyls, by photolysis in the presence of phosphine or phosphite. The substitution is often aided by a trace of (CpFe(CO)2)2, which is indicative of a radical (see Radicals) chain substitution mechanism. Phosphite ligand (as in 15) has been reported as being a particularly good replacement ligand from the standpoint of thermal stability.Nevertheless, neither C-3-substituted ()] -allyl)Fp complexes (14b) nor cyclic allyl complexes (see Allyl Complexes) may be made directly by this method the carbocyclic cases have been prepared by methoxide-induced proton abstraction of aUcene cation (16) (Section 4.3.2). " ... 

In this section (Section III) we have discussed substitution reactions of ligands in the first class of complexes (see Section II). Except for acetylene complexes, this class exhibits a tendency to react by an associative mechanism with increase of coordination number in the transition state. Data on monodentate olefins are in agreement with this suggestion. Substitution of polydentate ligands is more complicated and usually depends upon the nature of both the replacing ligand and of a fragment to be substituted. Thus substitution involves competition between both possible mechanisms. 

We will first consider ligand dissociation reactions. When coupled with addition reactions, dissociation reactions can be useful synthetically, providing an avenue to replace ligands such as carbon monoxide and phosphines by other ligands. 

Nitric oxide replaces ligands such as CO, tertiary phosphines, and alkenes as in the following reactions (20-22) ... 

Much of the work on ligand substitution of organometallic complexes has involved metal carbonyls with a trialkyl or a triarylphosphine serving as the replacement ligand. Two main mechanistic pathways exist by which substitution may occur— associative (A) and dissociative (D). 

The mechanism of this reaction is not as yet quite clear. Since cis/ trans isomerization of the replaced ligand is observed in the photochemical but not in the thermal substitution reaction, the mechanisms of these two reactions appear to be different 282>. The photochemical reaction must involve a species, which can rotate about the C=C bond. 

It is possible to place molecules or replace ligands, e.g. H2O by CH3OH [17], at different sites of the surface, in particular to replace deliberately up to two H2O ligands at the [Mos] groups (thereby changing the properties of the clusters) and to study direct reactions between the molecules placed inside the cavity. 

There have been several reports of reactions in which N2 replaces ligands other than water. The mechanism of these reactions is unknown, so that it is not clear whether we are dealing with an SnI or Sn2 type of reaction. In the reaction (19)... 

II) and (III) can be distinguished by reaction with a chelating agent (see Section 13.2). Chelating agents can replace ligands in a cis position, but not tram. 

Replacing ligand hydrogen by methyl groups or by ternary alkyl groups lowers volatility in coordinatively saturated complexes and increases volatility in complexes that are coordinatively unsaturated. 

Replacing ligand hydrogen with fluorine generally increases volatility, for instance, in j5-diketone alkyl groups. 

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