Second- and Third-Row Transition Metal Ions

Geometry Octahedral Octahedral Octahedral Octahedral Square planar Square planar 

Experimental data on water exchange on second- and third-row transition metal ions are much more scarce (Table 4.3). 

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Examination of the reaction kinetics of the M+ + H2S reactions show that these reactions are not simple first-order reactions, that is, nonlinear slope for the rate of disappearance of M+ shown in Fig. 7 for Pt+. The non-first-order rate of disappearance of M+ suggests that there is more than one intermediate, possibly due to the presence of electronic excited states of the metal ions or intermediates with different interactions between the metal and H2S. The addition of H2S to Au+ is similar to the reaction of H2S with Ag+ and Cu+ (M+ — [MH2S]+ — [M(H2S)2]+), but is dissimilar to most of the second- and third-row transition metal ions. 

Rate Constants and Activation Parameters for Water Exchange on Second and Third Row Transition Metal Ions... 

However, the reaction requires only a general acid catalyst rather than the specific acid catalyst H+, and the corresponding reactions of the soft thioether may be better mediated by softer Lewis acids such as Cu+, Ag+, Hg2+, Pd2+, Pt2+ or Au3+. In many cases the aqua-ted metal ion is the most convenient Lewis acid, but in the case of some metals, particularly the second and third row transition metal ions, the aqua ions are not isolable and other complexes (particularly those with chloride ligands) are equally effective. The role of these softer metal ions as Lewis acids is two-fold. Firstly, the sulfur is co-ordinated to the metal, which increases the polarisation of the C-S bond and enhances the electrophilic character of the carbon, and, secondly, the thiol (or thiolate) leaving group is stabilised by co-ordination (Fig. 4-39). 

Nefedov et al. (217) screened a variety of first-, second-, and third-row transition metal ions (i.e., V, Cr, Fe, Co, Ni, Cu, Mo, Rh, Pd, Ag, Ce, Hf, W, Pt, Au), impregnated as metal salts (0.5 wt. % metal) onto NaX. With the exception of rhodium none of these metal ions showed significant methanol carbonylation activity. 

III.B.2), complexes with manganese, chromium, as well as second- and third-row transition metal ions (e.g., ruthenium) oxidation reactions with dioxygen alone or with other peroxides (e.g., ferf-butyl-peroxide) the stabilization and spectroscopic characterization of mononuclear superoxo, peroxo, and oxo complexes other catalytic processes (e.g., the iron-catalyzed aziridination), enantioselective reactions with chiral bispidine ligands and the iron oxidation chemistry continues to produce novel and exciting results. 

Hagihara and coworkers in the 1970 s and early 1980 s have reported a successftil condensation polymerization strategy to incorporate late second- and third-row transition metal ions (mainly Pt(II) and Pd(II)) as part of a polymeric linear chain [65-67]. Since these metal ions prefer square-planar geometric structures, they designed compounds that contained two reactive chlorine groups in a trans orientation. Condensation of such di-fimctional monomers with traw -diacetylides afforded linear polymers which are calledpolyynes (Fig. 8.32). 

These ideas are consistent with the inertness of the octahedral complexes of Cr(III) low-spin Co(III) (tj/), Fe(II) (fj/) and Fe(III) and the inertness of the complexes of the second- and third-row transition metal ions with more than two d electrons, which are low spin. They also provide a simple explanation for the fact that the complexes of V(III) are more labile those of V(II) whereas the complexes of Cr(III) are inert and those of Cr(II) are labile. 

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... 

In summary, some coordination compounds are kinetically inert, whereas others turn out to be labile. Furthermore, this lability seems to be unrelated to the thermodynamic stability of the compound. Now, being a veteran chemistry student trained to ask critical questions, you are about to ask How can we tell which complexes will be inert and which will be labile As you might suspect, this is indeed a crucial question. It turns out that complexes of thefirst-row transition metal ions, with the exception of and Cr , are generally labile, whereas most second- and third-row transition metal ions are inert. But how, you ask, do we explain such a statement Why, for example, should the rates of reactions involving Co " and Cr be different from those involving other first-row transition metal atoms and cations What is it about these particular cations that makes them so inert To start to answer such queries, we now turn to a discussion of some of the most extensively studied reactions of coordination compounds, those involving the substitution of octahedral complexes. 

Labile and inert are kinetic terms that classify coordination compounds by how fast they react. Labile compounds react quickly and inert compounds slowly. These kinetic terms should not be confused with the thermodynamic terms stable and unstable. Compounds can be thermodynamically unstable but kinetically inert or, conversely, stable but labile. Complexes of the first-row transition metals, with the exception of Cr and Co, are generally labile, whereas coordination compounds of second- and third-row transition metal ions are inert. 

The photochemistry of dithiolene complexes of selected second- and third-row transition metal ions have also been investigated. Like the complexes with nickel, dithiolene complexes of palladium, platinum, molybdenum, and tungsten catalyze the formation of hydrogen from water when aqueous solutions of the complexes are irradiated at wavelengths shorter than 290 Tetrahydrofiiran is... 

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