Ligand intramolecular exchange

Diarylalkoxyiodinane (38) shows an intramolecular ligand exchange, which is thought to proceed through a 12-1-4 transition state or intermediate isoelectronic with XeF4, a 12-Xe-4 species, with a AG value of ca. 50.4 kJ mol-1 (82JA902). 

An explanation could be achieved by the consistent application of the mechanistic concepts used to explain data at tetravalent phosphorus compounds (48). Most of the available data on nucleophilic displacements are usually explained by an apical entry of the nucleophile displacement of the leaving group occurs from an apical position (Scheme 3). The first invoked TBP intermediate is the most stable one on the basis of the relative apicophilicity of the groups attached to phosphorus the more electronegative ligands prefer apical positions. Since both inversion and retention can proceed, the retention pathway has been rationalized on the basis of intramolecular ligand exchange by pseudorotation (Scheme 3). 

Bis(trifluoromethyl) A3-iodane 6a undergoes degenerate ligand exchange with added alkoxide PhC(CF3)2OK more rapidly (second-order rate constant = 49 M 1s 1 at 56 °C) than that of dimethyl A3-iodane 6b (second-order rate constant =61 M 1s 1 at 93 °C), in which an associative mechanism involving the formation of [12-1-4] species was proposed [16]. The CF3 substituents, which lower the electron density on iodine(III) relative to the CH3 substituents, make the iodine of 6a more susceptible to attack by alkoxide ion. Dynamic 19F NMR of A3-iodane 7 showed an intramolecular ligand exchange via intermediacy of bicyclic tetracoordinated iodate with a AG of ca. 12 kcal/mol at - 80 °C [17]. 

No structural data are available for the pentakis(trifluorophosphine) complexes [M(PF3)5] (M = Fe, Ru, Os), which almost certainly have trigonal-bipyramidal structures. Detailed 19F and 31P NMR studies indicate clearly that these molecules are fluxional even at temperatures as low as — 160°C and the barrier to intramolecular ligand exchange between equatorial and axial positions of the trigonal bipyramid is less than 20 kJ/mol (251). Figure 3 shows the temperature-dependent 19F NMR spectrum of [Ru(PF3)5], which is typical for all the MPS... 

Fig. 4. Possible intramolecular ligand-exchange mechanism in M(PF3)5 complexes.

The turnstyle mechanism is another possible way of an intramolecular ligand exchange. In the process of a turnstyle motion one of the axial ligands and one of the equatorial ligands exchange their places as if rotation about the local axis were of second order, the remaining three atoms rotating about the third order axis in the opposite direction... 

Fig. 3). Provided that this isomerization is based on an intramolecular ligand exchange, the free energy of activation for this process amounts to AG = 70 (4), 72 (5), or 70 (6) kJ mol . These data are in accordance with the results obtained in ab initio studies with the related bis[l,2-benzenediolato(2-)]- Fig. 2. hydrido(l-) ion (7) [2] (Fig. 2). 

Intramolecular Ligand Exchange of Zwitterionic Spirocyclic Bis [1,2-benzenediolato(2-)] organosilicates Ab Initio Studies of the Bis[l,2-benzenediolato(2-)]-hydridosilicate(l-) Ion... 

DiaUcyl as well as diaryl tellurides behave as monodentate ligands. Consistent with their soft character, their best explored group of complexes involves Pd(II) and Pt(II) centers. A variety of techniques have been applied to the characterization of their solid state and solution stractures. In such square-planar complexes, [MX2(TeR2)2] (M = Pd, Pt X = Cl, Br, I), cis - trans isomerization, intramolecular ligand exchange and tellurium inversion processes have all been observed by detailed far-infrared and variable-temperature NMR studies (see Refs. 14,15,17,3 5 for a more detailed review of the subject). 

The intervention of hexacoordinated intermediates in intramolecular ligand exchanges can be an alternative mechanism to pseudorotation. Hexacoordinated intermediates are involved in intermolecular ligand exchanges of pentacoordi-nated silicon compounds (286) and in the hexamethylphosphoric triamide (HMPA) catalyzed F-Cl exchange of tetracoordinated silanes (355). 

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