Nucleophilic Attack on the Ligands

When the ligand Is attached to a metal that is electron withdrawing, ligands that are typically electron-rich (such as CO, alkenes, and arenes) now 

Cyclometalation reactions resulting from /-elimination and d-eli mi nation reactions, respectively. 

Attachment of the arene ring to the electron-withdrawing metal preferentially directs the nucleophilic attack on the ring to the meta position. 

Examples of the DGM rules at work—nucleophilic attack occurs preferentially at even hapticity and open ligands. 

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Reversible changes in the electronic and NMR spectra of [Pt(bpy)2l in the presence of OH or MeO led to the suggestion that nucleophilic attack on the ligand was occurring (19, 71,305) in view of the formation of outer-sphere (494) and 5-coordinate complexes (557) by attack at the metal, these results are best reinterpreted in terms of nucleophilic attack at Pt (244, 677). 

The reaction shown in Eq. 12.67 gives complete inversion at the stereogenic center attached to Fe. This requires a backside attack, and the most logical way for this to occur is to start with an oxidative addition of the Br2, and then nucleophilic attack by bromide with Fe as the leaving group. The sequence of electrophilic addition of X2 to the metal followed by nucleophilic attack on the ligand is common for middle-to-late transition metals. Interestingly, when phenyl is in the 3-position, the reaction proceeds with retention. Retention is best explained by a double inversion, and the phenonium ion has been substantiated as the intermediate formed (Eq. 12.68). 

Increased back bonding to iv-bound ligands (e.g., Sections 5.1-5.3) weakens bonds within the ligand and decreases the tendency for nucleophilic attack on the ligand. 

Fig. 2 Stoichiometric nucleophilic attack on the terminus of an allyl ligand...

Nitrosyl complexes, in which Vno > 1886 cm or Fnq > 13.8 mdyn/A, usually react as electrophilic nitrosating agents so that the ligand can be considered NO [26]. Nucleophilic attack on the nitrosyl nitrogen is a common reaction encountered in the chemistry of nitroprusside and the rates and activation parameters for a number of different nucleophiles are listed in Table 3. Hydr-oxylamine adducts to nitroprusside via a rate law that is first order in the complex, the ligand and hydroxide (k = 4.5 x 10 M s ). 

The proposed mechanism for allyhc acetoxylation of cyclohexene is illustrated in Scheme 15. Pd -mediated activation of the allyhc C - H bond generates a Jt-allyl Pd intermediate. Coordination of BQ to the Pd center promotes nucleophilic attack by acetate on the coordinated allyl ligand, which yields cyclohexenyl acetate and a Pd -BQ complex. The latter species reacts with two equivalents of acetic acid to complete the cycle, forming Pd(OAc)2 and hydroquinone. The HQ product can be recycled to BQ if a suitable CO catalyst and/or stoichiometric oxidant are present in the reaction. This mechanism reveals that BQ is more than a reoxidant for the Pd catalyst. Mechanistic studies reveal that BQ is required to promote nucleophilic attack on the Jt-allyl fragment [25,204-206]. 

Electron-donating R groups, which decrease the positive charge on phosphorus, inhibit also phosphoryl formation, but also have another stabilizing effect, because they decrease the solvolysis or ligand-exchange reactions occurring by nucleophilic attack on the phosphorus. 

A similar structure is observed for the Pd(p-/ -PrPhDta)2PPh3 complex, where a PdS3P core is attained and both uni- and bidentate dithiocumate ligands are found (587). A mechanism involving nucleophilic attack on the OR group of the xanthate ligand by free xanthate ion is proposed (10) for the reaction of M(RXant)2 complexes with tertiary phosphines ... 

In an unusual sequence of reaction steps, nucleophilic attack of cyanide ion on a ligand results in cleavage of a sulfur-nitrogen bond.108 The resulting product then undergoes nucleophilic attack on the intermediate isothiocyanate in a reaction typical of the previous section. The sequence is summarized in Scheme 28. 

Cl, N02) undergoes cod substitutions with isocyanides and phosphines.190 However, when R is the strongly electron withdrawing N02 group, reaction with PEt3 leads to nucleophilic attack on the coordinated cod ligand affording (63) instead of cod substitution. 

C. Formation of Perfluorocyclooctadienetriyl Complexes of Iron by Nucleophilic Attack on the 1,2,3,6-ri4-OFCOT Ligand... 

The electron distribution in the vinylidene ligand, which results in a high electron deficiency on the a-carbon, but with considerable electron density on the / -carbon (see Section VII,A), renders this ligand subject to nucleophilic attack on the former, and electrophilic attack on the latter. 

A metal ion might play a number of roles in modifying the behaviour of co-ordinated electrophiles. Co-ordination of a donor atom X to the metal polarises the C-X bond and increases the electrophilic character of the carbon. In the case of an unsaturated centre this polarisation may be countered or even completely reversed by back-donation from filled (d) orbitals on the metal into vacant 7t -(or similar) orbitals of the ligand. The X- group generated after nucleophilic attack on the carbon atom may also be stabilised by co-ordi-... 

Ans. Ethylene to acetaldehyde, cyclohexane to adipic acid, p-xylene to tere-phthalic acid, and propylene to propylene oxide. For the first, redox chemistry of palladium and nucleophilic attack on coordinated ligand. For the second and third, radical chain, and for the fourth, oxygen atom transfer. 

Reactions involving nucleophilic attack on coordinated ligands are promoted by very polar solvents that stabilize anions. Metal carbonyls are also attacked by strong nucleophiles (RLi) to give the Fischer carbene complexes qv below. Section A 1.4),... 

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