Coordination sphere, vacancies

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. 

Fig. 7.13, this shifts the vacancy—represented by the square-in the coordination sphere of the titanium to a different site. Syndiotactic regulation occurs if the next addition takes place via this newly created vacancy. In this case the monomer and the growing chain occupy alternating coordination sites in successive steps. For the more common isotactic growth the polymer chain must migrate back to its original position. 

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. 

Two factors combine to lend a greater diversity in the stereochemistries exhibited by bivalent germanium, tin and lead compounds, the increased radius of Mn compared with that of Mw and the presence of a non-bonding pair of electrons. When the non-bonding pair of electrons occupies the isotropic valence level s orbital, as in, for example, the complex cations Pb[SC(NH2)2]6+ and Pb[antipyrine]6+, or when they are donated to conductance band levels, as in the binary tin and lead selenides or tellurides or the perovskite ternary phases CsMX3 (M = Sn, Pb X = Cl, Br, I), then the metal coordination is regular. However, in the majority of compounds an apparent vacancy in the coordination sphere of the metal is observed, which is usually ascribed to the presence of the non-bonding pair of electrons in a hybrid orbital and cited as evidence for a stereochemically active lone pair . 

Many of the organochromium compounds exist as dimers, e.g. diallyl-Cr(II), and one exists as a tetramer i.e., Cr(II)4tmsm8 (82,83). This is interesting in view of the assertion by some (2) that paired chromium is necessary for polymerization. In fact, neither species is very active for ethylene polymerization until it has been supported on a carrier. The monomeric organochromium compounds behave in about the same way. It seems likely that these polymeric chromium compounds react with the support to form isolated monomeric surface species, thus becoming coordin-atively unsaturated. When monomeric compounds react with the surface the loss of a ring or other multicoordinate ligand probably also leaves vacancies in the coordination sphere. 

Among the first 18-electron (18e) Fischer-type metal carbene complexes to be used as part of an olefin metathesis catalyst system were W[=C(OMe)Et](CO)5 with BU4NCI (for pent-l-ene)79, and W[=C(OEt)Bu](CO)5 with TiCLt (for cyclopentene)80. These complexes may also be activated thermally, e.g. for the polymerization of alkynes81, or photochemically, e.g. for the ROMP of cycloocta-1,5-diene82. The essential requirement is that a vacancy be created at the metal centre to allow the substrate to enter the coordination sphere. Occasionally the substrate may itself be able to displace one of the CO ligands. 

As the electronegativity of the E atom increases, the electron pair shared by the E(II) atom will be more contracted. Thus, the space occupied will be less. Bonds adjacent to lone pairs should experience the largest repulsions and become longer than those farther away. While long bonds are usually trans to each other, short bonds are usually trans to a vacancy in the coordination sphere. We infer the position of the lone pair from the nature of these geometric distortions. 

The sixth site of the Ti centers comprises defects, where some Ti centers lack their full complement of chloride ligands. The alkene molecule will bind at these vacancies. In ways that are still not fully clear, the alkene converts to an alkyl ligand group. The stereospecific nature of the alkane produced is facilitated by restriction of the coordination sphere of the titanium atom [12],... 

Case 2. The same system as in Case 1 is now added with a tertiary amine. The amine occupies a vacancy on the metal instead of the isocyanate. If the isocyanate is activated outside the coordination sphere of the metal, i.e. by a carboxylic ligand, the process takes place (Fig. 5a). 

The simultaneous formation of nitrites and hyponitrites can be imderstood on the basis of the redox properties of Ce ions. Thus nitrites may form when NO is adsorbed on Ce " " ions with anion vacancies in their coordination spheres, leading to reduction of the cerium cation ... 

In spite of its rigid structure in solution and the trans disposition of the hydride and the coordination vacancy, complex 1 reacts with terminal alkynes to afford alkenyl derivatives, as a result of the addition of the Ru-H bond to the carbon-carbon triple bond of the alkynes [9]. In all cases, the alkenyl ligands have an -stereochemistry and lie in the apex of a square pyramid similar to that of 1. This property along with the presence of the chloride ligand allows the entry to C[, C2, C4, and SH fragments into the coordination sphere of the ruthenium. 

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