Ligand lability

In the case of replacement of CO from the group VI carbonyl compounds there is additional evidence to the effect that the type A ligands labilize CO whereas the type B do not, but rather promote a second-order reaction. For the group VII octahedral compounds there is no strong evidence in favour of an associative activation step, except when interpretation is obscured by subsequent or concurrent reaction (but see ref. 146). There is, however, good reason to believe that such an associative reaction does occur in certain of the group VI compounds. 

The steric hindrance associated with the bis(3,5-dimethylpyrazol-l-yl)acetato ligand labilizes one of the PPha ligands in 24. Due to this labilization the reaction of [Ru(bdmpza)Cl(PPh3)2] (33a) with two... 

OCap Ligand Lability of ( iO)3TiOCap During the Reaction... 

In addition to the type of electron transfer reaction, shown in Equations 6.142-6.145, there are examples where pure MLCT excited states induce ligand substitutions by associative or dissociative mechanisms. A well-established example of a MLCT excited state-mediated ligand labilization reaction is shown in Equation 6.149.136... 

E. Infi.hence of the Phosphine Ligands and the Metal upon Nitrogen Ligand Lability and Basicity... 

Two extreme cases are possible (a) An entering olefin (this may be the second double bond of an already bonded 1,3-diene) labilizes the complex, and coupling results. In this case the entering olefin is involved in the reaction, (b) An accelerating ligand labilizes the complex, and coupling occurs between groups already bonded to the metal. A clear example of this is to be found in Eq. (43). 

The following order of ligand lability has been observed ... 

Knowledge of the energy-level diagram for the square pyramidal geometry allows us to rationalize immediately (22) the rates of ligand labilization in M(H20)62+ molecules (kt), determined in the classical... 

Whatever the nature of the photoreactive state, photolysis of a Rh]I1 amine leads to increased electron density in the metal-centered, a eg orbitals. Ligand labilization (especially of strongly cr-donating ligands), and subsequent solvolysis, is the anticipated (and observed) reaction. Photoinduced substitutions have now been reported for a large number of Rhm amines, and some of the results are summarized in Table 49. 

The electrochemical reduction of [Rh(bipy)2Cl2]+ and [Rh(bipy)3]3+ in room temperature acetonitrile solutions has been studied by DeArmond and co-workers and is represented in Scheme 29. For both complexes, they claim that the initial one-electron reduction is followed by a moderately fast elimination of a ligand (Cl- or bipy) if the cyclic voltammetry scans are sufficiently rapid, this reduction is reversible.824 After ligand labilization, the two paths merge, and there is evidence for two additional one-electron reductions, leading to [Rh(bipy)2] and [Rh(bipy)2]-. 

The current status of our understanding in this area has been succinctly summarized by Hoffman, who commented that. . reduction of [Rh(bipy),]3+ in aqueous solution yields a very rich chemistry.. . Both [Rh(bipy)3]2+ and [Rh(bipy)2]+ are involved in highly complex interlocking ligand-labilization, acid-base, redox, and aggregation reactions .825... 

The rate of axial ligand dissociation increases dramatically with the number of electrons added, being much faster (Eq. 4) for [Re(X)(CO)3(bpy)] than [Re(X)(CO)3(bpy)] X = Cl or Br. Five-coordinated species [Re(CO)3(bpy)] are formed. The same reaction occurs for other polypyridine ligands. Only [Re(Br)(CO)3(abpy ")] reacts via slow substitution of Br by a solvent (THF) molecule instead of dissociation, and full axial ligand labilization requires addition of one more (third) electron [136]. The complexes [Re(S)(CO)3(bpym)] S = PrCN, THF are also stable in solvent S even at room temperature [137]. For other polypyridines, the two-electron reduced solvento species [Re(S)(CO)3(N,N)] S = CH3CN, PrCN were observed [137] only at low temperature, in the solvent S, in equilibrium with [Re(CO)3(N,N)] . 

Electron-transfer Properties of Ground-state Polypyridine Complexes 833 5.3.5 Ligand Labilization... 

From this simplified scheme, it follows that the reductively-induced ligand labilization creates a vacant site to which a substrate A can coordinate. At the same time, the N,N ligand acts as an electron reservoir, accommodating electrons which are used to transform the coordinated substrate to Ared- A similar scheme can be drawn for one-electron substrate reduction following a ligand loss upon the first reduction. Indeed, both one- and two-electron catalytic reductions based on polypyridine complexes have been observed [135, 141],... 

The driving force for such high CO ligand lability probably rests with the ability of the heteroatom attached to Ccarbene to donate electrons by resonance to the metal, which becomes electron deficient upon loss of CO. 

Photosubstitution reactions and quantum yields of aq Rh(III) and Ir(IlI) complexes are summarized " in Tables 1 and 2. Each of these complexes has a ligand-field (LF) state as the lowest-energy excited state in HjO, and ligand labilization occurs with moderate to high quantum yields. In several cases, the antithermal pathways dominate, e.g., LF photolysis of the halopentaammines gives both the normal thermal reaction, i.e., halide aquation, and an antithermal pathway, ammine aquation ... 

As seen for Co(IIl) complexes, photolysis of disubstituted Rh(III) and Ir(lll) tetraammines often leads to photoisomerizations concomitant with ligand labilization, e.g. ... 

Pulse radiolysis is the radiation chemical analogue of flash photolysis. It is a fast-kinetics technique that enables transitory processes, initiated by the absorption of ionizing radiation, to be observed in time frames as short as the submicrosecond region. It permits the detection and characterization of short-lived intermediates, the determination of the kinetics of their decay, and a probing of reaction mechanisms. The technique finds use in the study of radiation effects on materials, and as a tool for the examination of mechanistic details. For inorganic systems, pulse radiolysis is used to characterize metal complexes in unusual oxidation states, to examine the kinetics and rates of ligand-labilization reactions and to elucidate the mechanism of electron transfer. 

The products of these reactions may be short lived, but they often have characteristic absorption spectra that can be detected by pulse radiolysis. Subsequent reactions, such as electron transfer and ligand labilization, can be followed kinetically with the appropriate detection technique. Reviews of the spectra, kinetics and mechanisms of complexes in unusual and unstable oxidation states are available A compilation of rate constants for the reactions of metal ions in unusual valency states is available ... 

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