Rigid coordinates

From the invariance of the coupling constant 2J(SiP) between HMPA-P and Si over a wide temperature range (compounds 4-7, 9-11) a rigid coordination of the HMPA to silicon can be deduced. Only in the case of the methyl complex 6 above 25 °C is the beginning of exchange of HMPA observed. However, a fast exchange of the coordinated acetonitrile at room temperature has been found for 12. 

Strapped tetramines show conformationally rigid coordination environments that can lead to novel stability properties. The coordination properties of bowl-shaped cavities or adamanzanes such as the ligand l,5,9,13-tetraazabicyclo-[7.7.3]nonadecane (95) were investigated.745 The... 

Figure 9.42 Secondary Building Units (SBU) using carboxylates with rigid coordination geometries that replace metal ions as vertices in MOFs. (a) Paddlewheel or lantern structure as in 9.17, (b) octahedral basic zinc acetate SBU used in MOF-5 and (c) a trigonal prismatic oxo-centred trimer. The poly-hedra use carboxylate carbon atoms as their vertices and the MOFs propagate via the linkers attached to these carbon atoms. The metal atoms are bound to only terminal ligands in addition to those shown.

Some substances such as quartz and feldspars melt as perfect crystals. The rigid coordination requirement of the Si+4 core does not permit these solids to develop a major concentration of vacant lattice sites. Albite melts at 1118° C., but such a crystal loses neither its shape nor its birefringence when held for several hours at a temperature 50° C. higher than it melting point (33). 

A bridging Mg2+ ion maintains rigid coordination patterns that stabilize in-line attack conformations... 

Figure 7. A metal ion-water-carboxylate motif found in many magnesium-utilizing enzymes. This motif probably helps to orient the fairly rigid coordination octahedron of magnesium.

The chemistry of 1,3-dicarbonyl substrates was subsequently extended further to the class of 2-cyano acetates [26]. This class of substrates is more challenging since their monodentate coordination behavior would suggest a less rigid coordination mode and hence a decrease in face differentiation. While initial exploration of the reaction with BINAP-Pd complexes such as 2-Pd led only to minor enantiomeric excesses of about 50% ee, Kim was able to devise a suitable protocol employing his cationic Pd complex [27] (Scheme 7). 

Under these conditions, the molecule is supposed to be in equilibrium with respect to the small amplitude vibration modes. As a result, the total energy of each conformation of the non-rigid coordinate space is determined by a reliable ab initio procedure with full optimization of the geometry with respect to the other coordinates. 

Fig. 25 a, b. If two locations on a rigid coordination polyhedron are fixed, the directions of binding of the other ligands is also fixed a 2 Mg + -O interactions fixed in an octahedral coordination scheme b 2 Zn + -S interactions fixed in a tetrahedral coordination scheme... 

A basic requirement for reaction (1) to proceed is that the two centers occupy predetermined positions, in a rather rigid coordinative framework, at the correct distance. This is the case of system 5 in which two tripodal tetramine subunits of the tren type are linked to an anthracene fragment that acts both as a spacer and as a fluorophore. 

Figure 1 Schematic representation of copper catenane Cu(l)" and free catenane (1). In Cu(l) the two highly rigid coordinating subunits fit together while encaging the metal atom, leading to pronounced intramolecular If-II interactions between the phen nucleus of a macrocycle subunit and one anisyl moiety of the other. In the free ligand (1), the two phen fragments are fully disentangled.

Obviously, the energy Vj, which is the potential energy surface for the nuclear motion, has a minimum value Vej at some equilibrium configuration Rea, where the set a comprises the rigid coordinates. In the vicinity of the minimum, the potential Vj is essentially quadratic, giving rise to a multidimensional harmonic oscillator. According to the standard theory, the frequencies of the harmonic oscillator scale as and the root-mean-square displacements from the Rea scale as k. Born and Oppenheimer have exploited the latter property by introducing new coordinates Qa for the displacement from equilibrium... 

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