Labile transition-metal ions, ligand

Ligand Substitution on Labile Transition-Metal Ions... 

Replacing one or more water molecules from the first coordination shell of a di- or a trivalent transition metal ion by a kinetically inert mono-or multi-dentate ligand can have a strong effect on the exchange rate constant of the remaining water molecules. In general the remaining water molecules become more labile (Tables VII and VIII) the acceleration can... 

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. 

DMPP itself is not a reactive diene in Diels-Alder reactions,but it is activated by coordination to transition metal ions. Complex 198 contains a labile perchlorato ligand that is easily displaced by the dienophile, which possesses a coordinating atom (O, S, As or P) in the group E. The cycloaddition reaction occurs intramolecularly in a highly organised environment, which leads to the coordinated exo cycloadduct 199 exclusively. A standard decoordination step affords the desired enantiopure ligands 200. Only the exo-syn isomers are formed, which bear the lone pair at the phosphorus atom and the dienophile functionalities at the same side of the molecule. ... 

Because of ligand-field factors, certain transition metal ions, notably Cr + and Co +, almost exclusively exhibit a coordination number of six in their complexes. The kinetically inert nature of Cr and Co complexes, dramatically different from that of the extremely labile lanthanide solvento ions, facilitates the isolation of isomeric species and was crucial in enabling Alfred Werner to formulate the fundamental tenets of coordination chemistry. For simple lanthanide ion complexes, their lability and the lack of a marked sensitivity of their visually observed colors to the nature of the coordination sphere renders Wernerian procedures inapplicable, such that the establishment of high and variable coordination numbers as a characteristic of lanthanide ions has depended largely on modem spectroscopic and crystallographic measurements. 

Most transition metal ions can be coordinated into a polymeric structure. These metal ions have empty or unsaturated d or f atom orbitals (receptors) that can accept electrons from ligand molecules (donors) to form coordination bonds. The first-row transition metal ions, such as, Fe +, Co +, Ni +, and Zn +, usually form labile coordination bonds with ligands in coordinating solvents such as water [23], whereas those of the second- and third-row transition metal ions often form irreversible coordination bonds [24], There are also exceptions, such as some lanthanoid... 

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