Mer-isomerism

As far as the fac/mer isomerization is concerned, the typical redox-induced conversion is that illustrated in Scheme 3 for complex [MnI(CO)3(dppm)Cl] (dppm = Ph2PCH2PPh2).10... 

Reversible fac-mer isomerization of the [LaCl3(HMPA)3] complex studied by variable temperature 1H NMR spectroscopy gave evidence for an associative mechanism [82], However it is not possible to distinguish between associative interchange mechanism and seven coordinate intermediate. An equilibrium mixture of [LaCl3(HMPA)3] and [LaCl3(HMPA)4] is also a possibility. 

According to the proposed mechanism, both exchange of [LaCl3(HMPA)3] with HMPA and fac-mer isomerization of [[LaCl3(HMPA)3] are second-order reactions and depend on the concentrations of the parent complex and free HMPA (hexamethyl phosphoramide). The proposed mechanism may be represented as follows. 

The proposed mechanism offac-mer isomerization and HMPA exchange in rLaCt HMPAE]. The central seven-coordinate species is either an interchange transition state, an intermediate or a stable molecule. 

There are many examples of redox-induced structural rearrangements in organometallic complexes the reader is referred to a review by Connelly. The simplest rearrangement is cis/trans or fac/mer isomerization, usually induced by oxidation. Thus, cw-[Mn(CO)2(dppe)2] slowly converts to the trans isomer with a rate constant of 10 s at room temperature. Upon oxidation, the cis — trans isomerization increases in rate by 7 powers of 10. The process is not catalytic, so that stoichiometric oxidation followed by reduction is required to synthetically utilize the increased reactivity of the radicals in the conversion of 18-electron cis-[Mn(CO)2(dppe)2l to frani-[Mn(CO)2(dppe)2]. An example of oxidatively induced fac mer isomerization is given in Scheme 10. The fac mer reaction for the neutral 18-electron isomer is slow, with 2 = 2 X 10 s , K2 = 4. The reaction fac mer is much faster and... 

The procedure can be modified to prepare the corresponding complexes of other amino acids. For example, if 11.0 g of alanine is substituted for the glycine and if, before addition of the acetic acid, the reaction mixture is heated for an hour after it has become violet, the yields of fac and mer isomere of [Co(H2NCH2CH2COO)3] are 1.5 g (11.6%) and 3.7 g (28.6%), respectively. 

M = Co) and obtained evidence far the existence of both fee- and mar-isomers. However, it was not possible to characterize these complexes properly because they are relatively insoluble in organic solvents. More recently, Srivastava and co-workers (8, 9) have prepared acetylacetone imine complexes of several trivalent metals, including Co(ni), but they have ignored the possibility of fac-mer isomerism. 

Isomerization of fac-Co(admh)q. The kinetics of fac-mer isomerization of Co(admh)3 have been studied by following the time dependence of the integrated intensities of the tort-butyl proton resonances (cf. Figure 1). Rate constants for fee to mer isomerization, k m, and equilibrium constants for mer to fee isomerization, Kmf are set out in Table V. The isomerization is first-order in the... 

Fac-mer isomerization (this work). —Optical inversion (ref 10). -Fac-mer isomerization (ref 23). —All errors are estimated at the 95% confidence level. 

Most substitutions at iron(III) are fast, and are therefore discussed elsewhere in this report (see Chapter 9), but several are slow enough to monitor by conventional techniques and are therefore mentioned here (though pentacyanoferrate(III) complexes are in Section 8.3.1). The first system bridges this and the preceding sections, for it involves relatively slow fac mer isomerization for tris-hydroxamato complexes of iron(II) and of iron(III). These complexes containing ligand (36) have been known for some time, but isomer details have only been sorted out in the course of the present kinetic study. Kinetics of formation of several iron(III)-hydroxamate complexes have also been reported. ... 

The power of time-resolved IR (TRIR) methods to solve mechanistic quandaries is superbly demonstrated in a study of the photochemical fac-mer isomerization of the Mn(Br)(CO)3(/Pr-DAB) complex (/Pr-DAB =N,N -di-rPr-1,4-diazabutadiene) (Scheme 6). Time-resolved visible absorption showed an initial species with Amax at 605 run formed within 400fs following excitation. This species decayed with an lips lifetime accompanied by the appearance of the product with identical kinetics. Based on this evidence alone, it was not possible to differentiate between two mechanistic interpretations (i) the 605 nm transient is an excited state and the 11 ps process is concerted CO loss and Br movement, or (ii) the 605 nm species is the CO loss primary photoproduct and the 11 ps process is the axial — equatorial Br movement. Subsequent experiments with picosecond TRIR showed that the IR bands of the 605 nm species were shifted to lower frequencies relative to the starting complex, a result interpreted as consistent only with the assignment of the species to the CO loss species and not the excited state. Note that a slower 22 ps process was attributed to coordination of a pyridine solvent molecule to the Mn(Br)(CO)2(/Pr-DAB) intermediate. 

Ligand topology and geometry Extra stability (chelate, macrocyclic, and cryptate effects) Topicity Conformational stereochemistry (coordination polyhedron) Geometric stereochemistry (cis—Irans and fac-mer isomerism) Chirality... 

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