Ligand substitution labile transition-metal ions

Ligand Substitution on Labile Transition-Metal Ions... 

The lability towards water exchange and ligand substitution generally of the first-row divalent transition metal ions increases in the sequence Cu2+ Cr2+ > Zn2+ Mn2+> Fe2+> -... 

Ligand Substitution in MA by the Porphyrin. Even in the most labile hydrated transition-metal ion, Cu2+, ligand substitution generally occurs more slowly than the preceding diffusion step (37). It is useful to look at reaction (8) in relation to the much faster reactions (9) and (10), which have been carefully studied by Diebler (38). 

The ligand substitution reactions of the bivalent first-row transition metal ions are the most studied of those of the labile metal ions, probably because the visible d-d spectra of the transition metal ions make them particularly amenable to spectrophotometric study, and also because their reaction timescale is usually well within those of the SF and NMR techniques. Thus it has been shown that the mechanism of dimethylformamide (dmf) exchange on [M(dmf)6] (M = Mn—Ni) varies systematically from L to D, in contrast to the analogous [M(solvent)6] in water, methanol, and acetonitrile where the mechanism varies from L h the number of d electrons increases. This has occasioned a spectrophotometric SF study of the closely related substitution of the bidentate ligands trans-pyndine-2-azo(p-dimethylaniline) (Pada) and diethyldithiocarbamate (Et2DTC) on [M(dmf)6] shown in Eq. (13) (where L-L represents a bidentate ligand) which... 

Copper(II) and zinc(II) are two of the more labile divalent metal ions and as a consequence the former is too labile for its water exchange rate to be determined by the NMR methods which utilize the paramagnetism of other divalent first-row transition metal ions, while the latter is diamagnetic and such NMR methods cannot be applied. However, it has been shown that water exchange rates and mechanisms can be deduced with reasonable reliability from simple ligand substitution studies, and this is one of the reasons for a recent variable-pressure spec-trophotometric SF study of the substitution of 2-chloro-l,10-phenanthroline on Cu(II) and Zn(II). The observed rate constants for the complexation reaction (kc) and the decomplexation reaction (k ) and their associated activation parameters for Cu(II) and Zn(II) are kc(298 K) = 1.1 x 10 and 1.1 x 10 dm mol" s", AH = 33.6 and 37.9 kJ mol", A5 = 3 and -2JK- mol", AV = 7.1 and 5.0 cm" mol", k 29S K) = 102 and 887 s", AH = 60.6 and 57.3 kJ mol", A5 = -3 and 4 J K" mol" and A V = 5.2 and 4.1 cm" mol". These data are consistent with the operation of an mechanism for the rate-determining first bond formation by 2-chloro-l,10-phenanthroline with the subsequent chelation step being faster [Eq. (18)]. For this mechanistic sequence (in which [M(H20)6 L-L] is an outer-sphere complex) it may be shown that the relationships in Eq. (19) apply. 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Solvent free methods have been used extensively in supramolecular chemistry, coordination chemistry and the formation of transition metal clusters and polymers. Reactions range from very simple ligand substitution reactions for salts of labile metal ions to more complex procedures, some of which are outlined below. 

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