Tetrahedral coordination centers

As mentioned previously, molecular mechanics force fields use redundant internal coordinates. In a tetrahedrally coordinated center such as Si04, all six bond angles Aa, , Aog are used, but only five of them are independent. Thus correlations exist between the force constants and the reference values of the coordinates, and this poses a problem in the fitting process because there is no unambiguously optimal set of parameters. Hence, parameters fitted to redundant coordinates may not be transferable and also may lack a well-defined physical meaning. 

In terms of double-stranded helices, the original examples were reported by Lehn et al. and involved chains of oligo-bipyridine ligands linked by the tetrahedral coordination center Cu(I) an example is shown in Figure 14 of the product formed in a self-assembly process. Other analogous longer (and shorter) double helices have been reported. " ... 

Boulon G (1987) Spectroscopy of post transition metal ions. In Di Bartolo B (ed) Spectroscopy of solid-state laser-type materials. Plenum Press, New York, pp 223-266 Boulon G (1997) Broad band centers applied for laser materials example of tetrahedrally coordinated centers, hi Di Bartolo B (ed) Spectroscopy and dynamics of collective excitations in solids. Plenum Press, London, New York, pp 561-573 Boulon G (2000) Transition metal ion lasers - Cr " ". In Webb C, Jones J (eds) Handbook of laser technology and application. Volume II. Laser design and laser systems. CRC Press, London, pp 307-337... 

From the data, the liquid is shown to have tetrahedrally coordinated aluminium with three chlorines and the isocyanate group attached. The neutron data clearly shows nitrogen, as opposed to sulfur, coordination to the aluminium center, forming an A1C13NCS species, which is consistent with a hard base/hard acid interaction as compared with the softer sulfur donation. It was also possible to show that a tetrahedral chloride environment is present around the lithium. 

Figure 39-14. Zinc fingers are a series of repeated domains (two to nine) in which each is centered on a tetrahedral coordination with zinc. In the case of TFIIIA, the coordination is provided by a pair of cysteine residues (C) separated by 12-13 amino acids from a pair of histidine (H) residues. In other zinc finger proteins, the second pair also consists of C residues. Zinc fingers bind in the major groove, with adjacent fingers making contact with 5 bp along the same face of the helix.

X-ray diffraction studies on several forms of the enzyme have demonstrated that the active site is composed of a pseudo-tetrahedral zinc center coordinated to three histidine imidazole groups and either a water molecule [(His)3Zn-OH2]2+ (His = histidine), or a hydroxide anion [(His)3Zn-OH] +, depending upon pH (156,157). On the basis of mechanistic studies, a number of details of the catalytic cycle for carbonic anhydrase have been elucidated, as summarized in Scheme 22... 

It should be noted that four-coordinate Cu(II) complexes do not typically favor tetrahedral coordination, so that distortions for this metal center may be more pronounced upon substitution at the 5-position. See Cotton, F. A. Wilkinson, G. Advanced Inorganic Chemistry (5th Ed.) Wiley-Interscience New York, 1988. 

The unit cell of cubic diamond corresponds to a face-centered packing of carbon atoms. Aside from the four C atoms in the vertices and face centers, four more atoms are present in the centers of four of the eight octants of the unit cell. Since every octant is a cube having four of its eight vertices occupied by C atoms, an exact tetrahedral coordination results for the atom in the center of the octant. The same also applies to all other atoms — they are all symmetry-equivalent. In the center of every C-C bond there is an inversion center. As in alkanes the C-C bonds have a length of 154 pm and the bond angles are 109.47°. 

Coordination studies of acyclic thioether ligands to silver(I) centers has also been studied. Poly(alkylthio)aromatic systems have been used to form supramolecular silver(I) compounds. With the ligand 2,3,5,6-tetrakis(isopropylthio)benzoquinone the compound has a linear chain structure in which silver(I) has a tetrahedral coordination.1156 A similar structure has the compound with the hexakis(methylthio)benzene,1157 but with the hexakis(tolylthio)benzene the silver... 

L-piperidine-2-carboxylic acid is a non-proteinogenic amino acid that is a metabolite of lysine. The zinc complexes of DL-piperidine-2-carboxylic acid, DL-piperidine-3-carboxylic acid, and piperidine-4-carboxylic acid have been studied. The X-ray crystal structures have been determined for the latter two. [ZnCl2(DL-piperidine 3-carboxylate)2] (42) is monomeric with a tetrahedral metal center and monodentate carboxylates. [Zn2Cl4(piperidine-4-carboxylate)2] (43) contains two bridging carboxylates in a dimeric structure. IR studies suggest that the DL-piperidine-2-carboxy-lato zinc has monodentate carboxylate ligands coordinating.392... 

Polymeric network structures of zinc and selenium with A.A -ethylenediamine have been formed and structurally characterized. The tetrahedral zinc centers are Se3N coordinated with the diamine bridging to give two-dimensional slabs 586... 

A novel cadmate complex has been formed by the reaction of Cd(NH2>2 with I C=CH in the presence of acetylene in liquid ammonia.250 The potassium salt, K2Gd(CCH)4-2NH3 191, has been structurally characterized. The cadmium center is tetrahedrally coordinated to four acetylide units with which it forms Cd-C bonds of 2.23-2.25 A (Figure 29). The acetylide ligands are 7r-coordinated to two crystallographically distinct potassium ions whose coordination sphere is completed by two ammonia molecules. 

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