Reactions at the Coordinated Ligand

In contrast to the highly reactive organoboranes, borabenzene metal complexes are surprisingly inert toward nucleophiles. However, cationic complexes may undergo nucleophilic addition reactions, and nucleophilic substitution has been observed with compounds having a hydrogen or an electronegative substituent at boron. 

Hydride addition to [CoCp(CsH5BPh)]+ (1) occurs at C-2 and C-4 as well as at the Cp ligand. The products 67 with an (r/5-l-bora-2,4- 

Quite in contrast to, e.g., [CoCp2]+, borabenzene metal cations show a pronounced affinity toward hard nucleophiles such as amines, OH-, and to some extent even F- and H20. Qualitatively this affinity increases in the order CoCp2]+ 36 1 61 (69). [CoCp(C5H5BPh)]+ (1) adds tertiary amines at boron. With pyridine, the pyridinioboratacyclohexadienyl complex 70 is formed (K = 174 5 liters mol-1, in MeCN, 20°C), which can be isolated from CH2C12 as PF6- salt (69). The similar rhodium and iridium cations 36 and 37 form the stable cyanide adducts 71 and 72 (69). 

More often, nucleophilic addition at boron can only be observed spectroscopically and is followed by more or less fast boranediyl extrusion (see Section VI,A) (69). A speculative mechanism has been proposed which links this boranediyl extrusion with the boranediyl insertion described earlier (Section V,A) (2,69). 

Related work, e.g., on Fe(CO)3[Et2C2(BI)2S] (99) and CoCp[XB-(CHCH)2BX] (X = OMe, Cl) (75), shows that nucleophilic substitution reactions certainly have a much wider scope than is apparent from the few examples known in borabenzene chemistry. 

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The above examples demonstrate the enormous synthetic potential of the electron-rich complex fragment Re(dppe)2 " which allows well-defined ligand substitutions in the axial positions and a variety of reactions at the coordinated ligands. This has also been observed for other ligands... 

All the complexes shown in equation (1) can undergo reactions such as ligand substitution (see Ligand Substitution) or oxidative addition (see Oxidative Addition), which reflect the character of the individual metal-ligand bonds. For metal-metal bonded complexes, reactions can occur at the following sites (1) reaction at the coordinated ligand (2) reaction at the metal-ligand bond (3) reaction at the metal-metal bond. 

The kinetics and mechanisms of template reactions are rarely studied in depth - they are often merely assumed to be genuine template reactions rather than a metal-activated reaction at a coordinated ligand. The best evidence for a true template mechanism is the isolation and characterization of intermediates with both reactants and with the macrocyclic product complexed with the template cation. 

Ligands in which phosphorus is the coordinating atom undergo a variety of reactions at the coordinated atom, and some of these have recently been discussed by Schmutzler 48). Platinum (II) complexes of trichlorophosphine solvolyze in water and alcohols to form phosphorous acid or stable orthophosphite esters. The gold compound, AuCKPCU), forms similar stable solvolysis products from alcohols but is reduced in water (13). 

This paper calls attention to the need for new ions in coordination chemistry—ions that would permit more detailed physico-chemical studies to be made, ions that would facilitate studies of less familiar metals and of less familiar coordination numbers, and ions that would help studies of chemical bonding and reaction mechanisms. Organometallic ions of the type RmM+ are such ions, and these form metal-chelate compounds of the type RmM Ch) . Three aspects of the chemistry of organometallic-chelate compounds are described (1) equilibria of compound formation ( ) kinetic and mechanistic studies of three types of reactions (a) reactions of the coordinated ligand, (b) substitution at the 4-, 5-, or 6-coordinate metal atom, and (c) reactions of the organic moiety and (3) studies of stereochemistry and chemical bonding. 

As discussed in Chapter 3, olefins and dienes bind to electron-poor metal centers by a flow of electrons from the olefin iT-system to the metal and from the metal to the olefin t -system. Thus, olefins bound to electron-rich and strongly backbonding metal centers react with protons and electrophiles directly at the metal-carbon bond. However, olefins and dienes coordinated to electron-poor metal centers are less reactive toward electrophiles than those bound to electron-rich metal centers or even free olefins and dienes. However, electron-poor olefin and diene complexes do imdergo reactions with electrophiles at the coordinated ligand by an indirect pathway. This indirect pathway occurs by insertion of the olefin or diene into the bond formed by attack of the electrophile at the metal. 

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. 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

In the o.s. reaction, the ion pair A+ - B is formed in a first step. The corresponding equilibrium constant can usually be obtained from simple electrostatic models. In this "ideal" case specific chemical interactions can be neglected and the rate constant of the E.T. step follows the theory of R.A. Marcus (see for example Marcus, 1975, or Cannon, 1980). In the i.s. reaction each of the three steps in reaction (9.2) may determine the reaction rates. The lability of the coordinated ligands at the... 

We shall consider the ways in which a metal may influence a reaction. These are listed in Table 6.1. The effects of the metal and the reactivity of the coordinated ligand are interrelated since invariably at least one of the reactants becomes coordinated to the metal during a catalyzed reaction. 

In pentacyano complexes of cobalt(III) the net charge at the coordination center is considerably decreased by the strong coordinate bonds of the five cyano groups. Thus the EPA properties of cobalt(III) are considerably lower in this complex unit, but further stabilization may still be effected by a sixth EPD ligand of high donicity like ammonia. By the reaction... 

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