Nucleophilic Attack at the Ligands

It usually occurs at the carbon atom of a linear carbonyl ligand. For example the nucleophilic attack of an OH group on a coordinated CO gives rise, via elimination, to the formation of anionic hydrido clusters  

This kind of attack is frequently encountered in water gas shift reactions catalyzed by molecular clusters. 

With mononuclear complexes one can define the oxidative addition of an X—molecule to a transition metal Mj by the simple equation 

One of the most important aspects of cluster chemistry is the possible 

THE RELATIONSHIP BETWEEN MOLECULAR CLUSTERS AND SMALL METAL PARTICLES 

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The shift of electrons away from the ligand, which is usually induced by the positive charge of the metal, will lead to accelerated nucleophilic attack at the ligand. Since nucleophiles donate electrons they thus act to redress the balance. This effect may well reinforce the neighboring group effect. Some examples of its operation are shown in Table 6.2. All the reactions are second-order. 

The coordination of a ligand to a metal can sometimes make the ligand itself more susceptible to nucleophilic attack. This is especially true when the metal has a high oxidation number and/or electron-withdrawing ligands. Some examples of nucleophilic attack at the ligand are shown in Figure 19.18. 

The complex [Ru(NH3)5(NaO)] + is amongst the products of reaction of [Ru(NH8)6(NO)] with hydroxylamine, hydrazine, or ammonia. The nitrous oxide complex is thought to arise from nucleophilic attack at the co-ordinated nitrogen. This type of nucleophilic attack at the ligand is an exact parallel of attack by, for instance, azide or hydrazine at the carbonyl carbon of a metal carbonyl (see Part IV, Chapter 5). 

Although surface organometallic chemistry is still in its infancy, there are already several examples of surface reactions leading to well-defined surface complexes (Table l-I). It appears that these reactions obey the same principles as those encountered in molecular chemistry nucleophilic attack at the ligands, electrophilic attack of the metal-carbon bond, oxidative addition, Lewis acid-base adduct formation, redox reactions, disproportionation, and the cooperative effect of dual acid-base sites in an insertion reaction. 

Mononuclear acyl Co carbonyl complexes ROC(0)Co(CO)4 result from reaction of Co2(CO)8 with RO-.77 These also form via the carbonylation of the alkyl precursor. The ROC(0)Co(CO)4 species undergo a range of reactions, including CO ligand substitution (by phosphines, for example), decarbonylation to the alkyl species, isomerization, and reactions of the coordinated acyl group involving either nucleophilic attack at the C or electrophilic attack at the O atom. 

The next oxidation step yields [(oep)Pb(IV)]2+, which is easily demetallated by nucleophilic attack at the lead atom followed by protonation of the ligand ... 

The mechanism proposed72 involves initial nucleophilic attack at the carbene carbon by the dithiocarbamate anion, effectively resulting in addition across the metal-carbon bond. Rearrangements of the dithiocarbamate ligands then form an V-allyldithiocarbamate species Complex 53 was isolated from the reaction mixture of 51 with the diethyldithiocarbamate and identified by X-ray crystallography. 

The breaking of carbon-to-phosphorus bonds is by itself not a useful reaction in homogeneous catalysis. It is an undesirable side-reaction that occurs in systems containing transition metals and phosphine ligands and that leads to deactivation of the catalysts. Two reaction pathways can be distinguished, oxidative addition and nucleophilic attack at the co-ordinated phosphorus atom (Figure 2.35). 

The LUMO in d pentatetraenylidene complexes is predominantly localized on the odd carbon atoms and to a lesser extent on the metal. The coefficients on Cl and C3 are very similar, independent of the metal-ligand fragment and the terminal substituent. The coefficient at C5 is somewhat larger. In square-planar d rhodium and iridium complexes the coefficient at the metal is comparable to that on C5 and is larger than those on Cl and C3. Thus, a nucleophilic attack at the metal of d complexes has also to be taken into account. 

In the postulated transition state, the y-phosphorus atom is penta-coordinated, whereby the ligands are configmed in the form of a triagonal bipyramid. Mg is indispensable for the catalysis it is needed for binding of substrate and product, as well as for the catalysis itself. Activation of the water molecule for nucleophilic attack at the y-phosphate requires involvement of side groups of the protein in the sense of a general base catalysis. 

Alkenes bonded to platinum(II) can be displaced by strongly coordinating ligands such as cyanide ion or tertiary phosphines. The displacement of ethylene from Zeise s salt by phosphines is a useful method of preparation of complexes trawa-PtCl2(PR3)2.711 Amines will also displace alkenes from coordination to platinum(II), but this reaction can compete with nucleophilic attack at the coordinated alkenic carbon. The stability of platinum(II) alkene complexes follows the sequence C2H4 > PhCH=CH2 > Ph2C=CH2 555 Ph(Me)C=CH2.712... 

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

Terminal monoalkenes were alkylated by stabilized carbanions (p a 10-18) in the presence of 1 equiv. of palladium chloride and 2 equiv. of triethylamine, at low temperatures (Scheme l).1 The resulting unstable hydride eliminate to give the alkene (path b), or treated with carbon monoxide and methanol to produce the ester (path c).2 As was the case with heteroatom nucleophiles, attack at the more substituted alkene position predominated, and internal alkenes underwent alkylation in much lower (=30%) yield. In the absence of triethylamine, the yields were very low (1-2%) and reduction of the metal by the carbanion became the major process. Presumably, the tertiary amine ligand prevented attack of the carbanion at the metal, directing it instead to the coordinated alkene. The regiochemistry (predominant attack at the more sub-... 

Nucleophiles partition between the two mechanisms based on their hard-soft characteristics, with soft nucleophiles undergoing ligand attack and hard nucleophiles attacking at the metal. A limited class of nucleophiles appear capable of adding by either mechanism, with secondary factors controlling their choice of mode of addition. 

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