Masking by coordination

In order to give a physical interpretation of special relativity it is necessary to understand the implications of the Lorentz rotation. Within Galilean relativity the three-dimensional line element of euclidean space (r2 = r r) is an invariant and the transformation corresponds to a rotation in three-dimensional space. The fact that this line element is not Lorentz invariant shows that world space has more dimensions than three. When rotated in four-dimensional space the physical invariance of the line element is either masked by the appearance of a fourth coordinate in its definition, or else destroyed if the four-space is not euclidean. An illustration of the second possibility is the geographical surface of the earth, which appears to be euclidean at short range, although on a larger scale it is known to curve out of the euclidean plane. 

The masking of the normal reaetions of simple ligands, such as the nitro, cyano, and ammonia groups, by coordination to a metal is a phenomenon encountered early by a chemist. One of the first examples of masking in a chelate complex was reported, signifieantly, in biological journals. It involves the protection by copper ion of the a-amino group in ornithine and lysine ... 

It is known that proton chemical shifts are largely determined by electronic effects The coordinate Si<-D interaction, weakening the covalent silicon bonds (increasing their ionic character), enhances the electron density on the atoms of a hydrocarbon substituent which results, as a rule, in an increase of proton shielding. This is usually accompanied by a drahielding of protons of the donor fragment This chemical shift contribution, however, is often insignificant and masked by other effects, in particular, when the silicon atom has a 7t-donor substituent. 

Attempts were then made to perform asymmetric catalytic reactions using chiral Lewis acid catalysts [59]. Reaction of the nitrone 73 and the oxazolidinone 76 with 10 mol % of the bis(oxazoline) 12-Mg(II) catalyst, prepared by Corey s method [13], in the presence of 4-A molecular sieves afforded the cycloadduct 77 in high yield (>95 %) and high (> 95 %) endo selectivity and 82 % ee (Sch. 33). The presence of activated powdered 4-A molecular sieves was essential to the endo and enantioselec-tivity of the reaction in their absence they were 65 % and < 2 %, respectively. The reaction proceeded via an intermediate XXIX, proposed by Corey [13], in which the bis(oxazoline) ligand 12 and the oxazolidinone 76 are both bidentately coordinated to the magnesium and addition to the re face is favored because the si face of the bound oxazolidinone is masked by one of the phenyl substituents on the oxazoline rings. 

For Ru(trpy)(py)32+ this process can be assigned to the pyridine localized reduction (E = -1.96V) by analogy with that in Ru(py)62+ (En c = -1.93V) (22). Both Ru(HC(pz)3)(vpy)32+ and Ru-(trpy) (vpy) 3" show an irreversible reductive wave at potentials 400 and 200 mV more positive, respectively, which can reasonably be assigned to the direct reduction of coordinated 4-vinylpyri-dine. In the case of complexes such as Ru(bpy)2(vpy)22+ or Ru-(bpy)2(stilb)22+, however the vinyl ligand reduction is intensely masked by a second bpy-4>py reduction process. 

Our view of the FeMoco changed drastically in 2002. Crystallographic analysis to 1.16 A resolution of A. vinelandii MoFe protein shows that there is a light atom X (C, N, or O) between the six iron atoms, whose presence had been masked by systematic termination errors that are inherent in Fourier analysis of diffraction data. " This discovery is consistent with numerous data that are more difficult to explain with the three-coordinate model. First, each of the six... 

The results of numerical fits to the spectra of octahedrally coordinated vanadium(II) and chromium(III) compounds are compiled in Tables 3 and 4, respectively. The fitting method employed is indicated, with reference to section III. B, in column 2. Procedure (d) does not always yield a real B value and it did not, in fact, produce a reasonable value where it is not listed. Thus, e. g., S = 892.9 cm and B= 1002.6 cm i are obtained by method (d) in Al203 Cr3+ and in emerald Cr +, respectively. Procedure (e) has not been considered at all, since it is not useful in compounds. Here, the first and second absorption band is, in general, easily accessible, whereas the third band is often masked by strong uv absorption. 

The catalytic activities oftm are determined at constant phosphine to metal ratio, deliberately kept high on the polymer to minimize leaching of the metal from the support. (The problem of physical and chemical loss of metal complexes from polymer supports is review in the chapter by Garrou.) High phosphine to metal ratios reduce the fraction of coordinatively unsaturated sites reduce activity. In some cases higher activity of polymer-bound, more coordinatively unsaturated complexes may have been masked by diffiisional limitations, discuss in the chapter by Ekerdt. 

---