Crystal coordination sphere

In typical organic crystals, molecular pairs are easily sorted out and ab initio methods that work for gas-phase dimers can be applied to the analysis of molecular dimers in the crystal coordination sphere. The entire lattice energy can then be approximated as a sum of pairwise molecule-molecule interactions examples are crystals of benzene [40], alloxan [41], and of more complex aziridine molecules [42]. This obviously neglects cooperative and, in general, many-body effects, which seem less important in hard closed-shell systems. The positive side of this approach is that molecular coordination spheres in crystals can be dissected and bonding factors can be better analyzed, as examples in the next few sections will show. 

Crystal structure of solids. The a-crystal form of TiCla is an excellent catalyst and has been investigated extensively. In this particular crystal form of TiCla, the titanium ions are located in an octahedral environment of chloride ions. It is believed that the stereoactive titanium ions in this crystal are located at the edges of the crystal, where chloride ion vacancies in the coordination sphere allow coordination with the monomer molecules. 

The crystal structure of the adduct of titanium tetrachloride and the ester formed from ethyl 2-hydroxypropanoate (ethyl lactate) and acrylic acid has been solved. It is a chelated structure with the oxygen donor atoms being incorporated into the titanium coordination sphere along with the four chloride anions. 

In the crystallization of these hydrated salts from aqueous solutions it is essential that a low pH (high level of acidity) is maintained, otherwise hydrolysis occurs and yellow impurities contaminate the products. Similarly, if the salts are redissolved in water, the solutions turn yellow/brown. The hydrolytic processes are complicated, and, in the presence of anions with appreciable coordinating tendencies, are further confused by displacement of water from the coordination sphere of the iron. However, in aqueous solutions of salts such as the perchlorate the following equilibria are important ... 

The three iridium complexes 72d, 72f and 72g were analyzed by X-ray diffraction. Unfortunately the iridium complex 72a, the most efficient in many reactions, failed to give suitable crystals for analysis but the corresponding crystalline rhodiiun complex 73 coifld be analyzed. According to the results obtained, the coordination sphere of the Ir atom and of the Rh atom can be described as pseudo-square planar (Fig. 12). 

The kind of bond between neighboring atoms also has to be considered. For instance, the coordination number for a chlorine atom in the CC14 molecule is 1 when only the covalently bonded C atom is counted, but it is 4 (1 C + 3 Cl) when all atoms in contact are counted. In the case of molecules one will tend to count only covalently bonded atoms as coordinated atoms. In the case of crystals consisting of monoatomic ions usually only the anions immediately adjacent to a cation and the cations immediately adjacent to an anion are considered, even when there are contacts between anions and anions or between cations and cations. In this way, an I- ion in Lil (NaCl type) is assigned the coordination number 6, whereas it is 18 when the 12 I- ions with which it is also in contact are included. In case of doubt, one should always specify exactly what is to be included in the coordination sphere. 

Cl- ions that form two square faces of different sizes. Pb2+ ions are located under one half of the squares of the F- ions an equal number of Pb2+ ions are situated above the other half of the squares, which in turn form the base faces of further antiprisms that are completed by another layer of Cl- ions. In this way, the total number of Pb2+ ions is the same as the number of F- ions the number of Cl- ions also is the same because there are two Cl layers for every F- layer. Together, these layers form a slab that is limited by Cl ions on either side. In the crystal these slabs are stacked with staggered adjacent Cl-layers. As a consequence, the coordination sphere of each Pb2+ ion is completed by a fifth Cl- ion (dashed in Fig. 7.5). 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

Thermolysis of 1-phenyl-3,4-dimethylphosphole in alcoholic solvents in the presence of NiCl2 leads to the synthesis of the racemic biphospholene complex (222).653,654 Upon reaction of the bromo derivative with AgBF4, the meso and racemic diastereomers of (223) are formed, which can be separated by fractional crystallization.655 In both (222) and (223) the coordination sphere is slightly distorted from square planar. 

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