Coordination sphere, expansion

Upon comparison of the k< m exchange rate of the Tc(V) system with that of the Re(V), the significant increase in reactivity (ca. 3 orders of magnitude) is very prominent and not necessarily indicative of an associative activation. It is, however, possible that the Tc(V) hydroxo complex might be very reactive via an associative pathway, since it is known that the Tc(V) center much more readily accepts electron density than does the corresponding Re(V) complexes (55). The greater ease by which coordination sphere expansion can occur in third-row d-series transition elements such as W(IV) and Re(V) (not very easily... 

From a coordination viewpoint, there are two views that support the idea of an inner-sphere coordinated cyano bridge on the Cr(VI) core. First, four-coordinated Cr(VI) or Cr(V) changes to six-coordinate Cr(IV) so that there is room for coordination sphere expansion. Second, it has been shown (118) that, in spite of the high negative charge on octa-and hexacyano complexes, they can be used as nucleophiles in substitution of an axial water ligand of mesotetra(4-A/ -methylpyridyl)por-phinediaquocobalt(III) ions and that the stability constants of these products are comparable to those of NCS . 

There has been recent expansion of work in this area with major contributions from Vahrenkamp, Parkin, Corrano, and their co-workers. The main aim has been to reproduce the coordination sphere of the enzyme-bound zinc.2,335 900-902... 

The enhanced tendency of the heavier chalcogens (S< coordination spheres, either by donor-acceptor interactions with Lewis acids or bases, or by intermolecular association, allowed to characterise crystallographically in recent years a considerable amount of supramolecular structures. 

II.B.2. Lewis Acid Sites and Expansion of Coordination Sphere. 28... 

With the Ti4+ ions acting as Lewis acid centers, a strong interaction with ammonia and water with these centers is expected. There is in fact abundant spectroscopic evidence for the coordination of NH3 and H20 molecules to tetrahedral Ti4+ centers and for the corresponding expansion of their coordination spheres. 

As indicated in Scheme 11, there are basically two classes of chemistry that have been observed for [Os CO) ]2-. One involves expansion of the ligand coordination sphere, without changing the stereochemistry of the metal cluster, and is electronically related to the parent carbonyl Os5(CO)18, while the second involves redox reactions, with addition of two electrons to the metal cluster and concomitant structural changes in the metal polyhedra (204). 

The common motif shared by non-heme iron oxygenases contains an active site, where two histidines and one carboxylate occupy one face of the Fe(ll) coordination sphere. These enzymes catalyze a variety of oxidative modification of natural products. For example, in the biosynthesis of clavulanic acid, clavaminic acid synthase demonstrates remarkable versatility by catalyzing hydroxylation, oxidative ring formation and desaturation in the presence of a-ketoglutarate (eq. 1 in Scheme 7.22) [80]. The same theme was seen in the biosynthesis of isopenicillin, the key precursor to penicillin G and cephalosporin, from a linear tripeptide proceeded from a NRPS, where non-heme iron oxygenases catalyze radical cyclization and ring expansion (eq. 2 in Scheme 7.22) [81, 82]. 

Fig. 9.4. The thermal expansion of bonds averaged over each coordination sphere as a function of bond valence. The circles are measured values, the lines represent eqn (9.15). The filled circles are for grossular (24944) discussed in the text.

Just as expansion of the coordination sphere by transition from the low-spin to the high-spin state enhances isomerization and racemiza-tion, so too should it enhance the rate of ligand substitution. Unpublished observations on the ligand substitution of [Fe(pyim)3]2+, which is coupled with its spin equilibrium, indicate that it is the high-spin state which preferentially undergoes the substitution reaction, as expected (117). 

The tendency to expansion of the coordination sphere of M, which increases when going from Ge to Sn, Pb. 

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