First transition series, substitution

Fibrillin, calcium binding, 46 473, 474, 477 Fibulin-I, calcium binding, 46 473 Field desorption mass spectroscopy, 28 6, 21 Field effects, of astatophenols, 31 66 Fine structure, 13 193-204 Fingerprinting of polymetalates, 19 246-248 Finite perturbation theory, 22 211, 212 First transition series, substitution, transferrins, 41 423 26... 

Metallocenes. — With the exception of the 4d6 system, ruthenocene, Ru(Cp)2, all the metallocenes for which adequate electronic spectra are available belong to the first transition series. For the 3d series metallocenes (and 1,1 dimethylmetallocenes) are known as neutral, Af(Cp)2, species for the elements from vanadium to nickel inclusive, whilst the cationic, M(Cp)2+, systems are found for chromium, iron, cobalt, and nickel, these being described generically as metallicenium salts. A number of substituted ferricenium species have also been reported and studied, including the 1,1 dimethyl derivative, but few spectro-... 

Differences between the two sites become more pronounced for metal ions other than Fe " and anions other than COa . The differences are most pronounced for larger metal ions such as lanthanides. For transferrin some of the larger lanthanides appear to bind in only one of the two sites (Section IV.B.3), and for lactoferrin, although binding occurs in both sites, the second metal ion binds much more weakly, as shown by the curvature of the UV difference titration graph (Fig. 18) the biphasic release of Ce from lactoferrin contrasts with that of Fe (Fig. 28). Even metal ions of the first transition series, of similar size to Fe "", enhance the differences between the two sites. When Cr is bound to either transferrin 134) or lactoferrin (154), different EPR signals are seen for the two sites, and one Cr " ion is much more readily displaced by Fe than the other. Likewise, the EPR spectra of VO " -substituted transferrin indicate different metal configurations in the two sites (207), as do NMR studies of Co -substituted ovotransfer-rin (139). In these cases one metal ion is also released much more readily than the other as the pH is lowered. 

Transition metal carbonyl hydrides of the first transition series undergo unusually rapid substitution reactions, e.g., [HMn(CO)s] is much more reactive than [ClMn(CO)5]. Carbonyl hydrides of most heavier transition metals are not unusually labile. Pearson proposes the hydride migration mechanism shown in Scheme 1 to account for the unusual lability. For this... 

Taking into consideration the deuterium isotope effect (k Yfy/kxfD) = 2.3), they concluded that fca< k a< /cb and that the rate-determining step was the first substitution. Ligand substitution was thought to proceed by the Ia mechanism, on the basis of the negative AS and the independence of the rate on the concentration of acetylacetone. This feature is compatible with the results with tris(acetylacetonato)metal(III) previously obtained [21]. Furthermore, in the second transition series the kt value decreases in the order... 

Thus far, in the alkaloid series discussed, the nitrogen atom has always been part of the core of the alkaloid structure, rather than acting in a dipolarophilic manner in the cycloaddition of the carbonyl ylide. Recently, Padwa et al. (117) addressed this deficiency by conducting model studies to synthesize the core of ribasine, an alkaloid containing the indanobenzazepine skeleton with a bridging ether moiety (Scheme 4.57). Padwa found that indeed it was possible to use a C = N it-bond as the dipolarophile. In the first generation, a substituted benzylidene imine (219) was added after formation of the putative carbonyl ylide from diazoketone 218. The result was formation of both the endo and exo adduct with the endo adduct favored in an 8 1 ratio. This indicates that the endo transition state was slightly favored as dictated by symmetry controlled HOMO—LUMO interactions. 

Inspection of Fig. 8 suggests that the discrepancy in the additivity relationship for multiple substitutions is most serious when the first substituent is strongly ED or EW (s-ED, s-EW). A transition state substituted by a s-EW group is more electron deficient than it is in the unsubstituted (Y = H) series, so that the electron demand in such a transition state must be more than that in the parent system. Consequently, the p-value for the Y = P-NO2 series is larger than that for the Y = H series. On the other hand, when the Y-substituent is s-ED, such as p-methoxy, then a smaller p results for the series. From these arguments, it is apparent that a change in the extent of electron deficiency in the transition state must be a function of the ED or EW power of the first substituent. TTiis effect can be expressed quantitatively by (6) and (7). 

In the meantime, certain approximations are in order. First, substitution of reasonable values of Csd and ( 3 and 30 kK, respectively) in the expressions for 6 and indicates that 6 is primarily a function of C and is primarily a function of zl. Now the optical transitions of ions which we might hopefully expect to observe will include ones centered at 0 and 20 in the first half of a transition series (n < 5) but ones centered at (f) — 6 and in the second half (n > 5), because of the electron-hole equivalence. Thus many of the transitions of 5d ions, where n < 5, can be treated as a function of spin-orbital and interelectronic interactions and many of the transitions of d, where n > 5, can be treated as a function of crystal field and interelectronic interactions. 

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