Tetrahedral to octahedral coordination

Recently Desimoni et used the same bis(oxazoline) ligand in the magnesium(II) catalysed Diels-Alder reaction of the N-acyloxazolidinone depicted in Scheme 3.4. In dichloromethane a modest preference was observed for the formation of the S-enantiomer. Interestingly, upon addition of two equivalents of water, the R-enantiomer was obtained in excess. This remarkable observation was interpreted in terms of a change from tetrahedral to octahedral coordination upon the introduction of the strongly coordinating water molecules. 

Germanium dioxide is an interesting example. Its radius ratio (Table 6-2) is 53 pm/140 pm = 0.38. This value is very near the transition value, 0.37, from tetrahedral to octahedral coordination, and GeOa is in fact dimorphous, with one crystalline form having the quartz structure (ligancy 4) and the other having the rutile structure (ligancy 6). 

While it remains true that tetrahedral and octahedral coordination modes are the predominant stereochemistries adopted by the group 13 metals, nevertheless increasing diversity is being achieved by carefully selecting appropriate electronic and geometric features to enhance the stabilization of unusual stereochemistries. Some representative examples follow. 

The successful rationalization of these transition-metal inverse spinel structures in terms of the relative LFSE s of tetrahedral and octahedral sites is another attractive vindication of ligand-field theory as applied to structure and thermodynamic properties. Once again, however, we must be very careful not to extrapolate this success. Thus, we have a clear prediction that LSFE contributions favour tetrahedral over octahedral coordination, except for d" with n = 0, 5 or 10. We do not expect to rationalize the relative paucity of tetrahedral nickel(ii) species relative to octahedral ones on this basis, however. Many factors contribute to this, the most obvious and important one being the greater stabilization engendered by the formation of six bonds in octahedral species relative to only four bonds in tetrahedral ones. Compared with that, the differences in LSFE s is small beer. Why , one asks, was our rationalization of spinel structures so successful when we neglected to include consideration of the bond count The answer is that cancellations within the extended lattice of the spinels tend to diminish the importance of this term. 

Even when forward reactions proceed rapidly at laboratory conditions, as is observed with Se(IV) and Cr(VI) reduction, evidence exists that chemical and isotopic equilibrium are not approached rapidly. Altman and King (1961) studied the kinetics of equilibration between Cr(III) and Cr(Vt) at pH = 2.0 to 2.5 and 94.8°C. Radioactive Cr was used to determine exchange rates, and Cr concentrations were greater than 1 mmol/L. Time scales for equilibration were found to be days to weeks. The mechanism of the reaction was inferred to involve unstable, ephemeral Cr(V) and Cr(IV) intermediates. Altman and King (1961) stated that the slowness of the equilibration was expected because the overall Cr(VI)-Cr(III) transformation involves transfer of three electrons and a change in cooordination (tetrahedral to octahedral). Se redox reactions also involve multiple electron transfers and changes in coordination. 

For cubic structures with more than one interionic distance—as, for instance, spinels (multiple oxides of type AB2O4 with A and B cations in tetrahedral and octahedral coordination with oxygen, respectively)—it is stiU possible to use equation 1.109, but the partial derivatives must be operated on the cell edge, which is, in turn, a function of the various interionic distances (OttoneUo, 1986). 

This treatment enhances the intensity of the octahedral aluminum signal for the low Si/Al ratio. Table II shows the percentage of Al corresponding to tetrahedral and octahedral coordination in each case. 

The steady-state emission of beryl has been previously studied. The broad band at 720 nm is connected with Fe ", while the relatively narrow bands at 480 and 570 nm are ascribed to Mn " in tetrahedral and octahedral coordination, respectively. Cr " emission was connected with narrow i -lines at 680 and 682 nm (Tarashchan 1978 Kuznetsov and Tarashchan 1988). 

Figure 4 shows the27 A1 NMR spectrum of a calcined sample. There are three peaks visible, a peak due to octahedrally coordinated aluminum (Oh), a peak due to tetrahedrally coordinated aluminum (Td) and a peak in between due to highly distorted tetrahedral sites. The tetrahedrally coordinated aluminum can be assumed to be incorporated into the aluminosilicate network while the octahedrally coordinated aluminum is occluded in the pores or exists as an amorphous by product. 

The electronic transitions of silicalite and TS-1 in the UV-visible spectrum have provided significant information about the structure of TS-1. The diffuse reflectance spectra of the two materials (Fig. 11) show a strong transition at 48,000 cm-1 that is present in the spectrum of TS-1 and absent from that of silicalite. This transition must be associated with a charge-transfer process localized on Tiiv. The frequency of this transition is modified by the presence of H20 (Fig. 12). As the H20 partial pressure increases, the peak at 48,000 cm- is progressively eroded with formation of a lower-frequency absorption, which reaches a new stable maximum value at 42,000 cm. These frequencies come very close to those that can be calculated by the Jorgensen equation for Tiiv tetrahedrally and octahedrally coordinated to oxygen, respectively. Furthermore,... 

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