Alkyls coordination numbers

Another reaction in the last step is the syn elimination ofhydrogen with Pd as H—Pd—X, which takes place with alkyl Pd complexes, and the Pd hydride and an alkene are formed. The insertion of an alkene into Pd hydride and the elimination of, (3-hydrogen are reversible steps. The elimination of, 3-hydrogen generates the alkene, and both the hydrogen and the alkene coordinate to Pd, increasing the coordination number of Pd by one. Therefore, the / -elimination requires coordinative unsaturation on Pd complexes. The, 3-hydrogen eliminated should be syn to Pd. 

The tetraorganotin compounds, R4Sn, show no tendency to increase their coordination number, owing to their weak, Lewis acidity conferred by the four electron-releasing alkyl groups. It has, however, been claimed (353) that trimethyl(trifluoromethyl)tin forms a 1 1 adduct with hexamethylphosphoric triamide, and that this may be isolated in the solid state. 

There are over 140 zinc alkyl or aryl structures reported in the CSD in which more than half of the donors are noncarbon. Those selected for discussion show representative structures and coordination numbers, or particularly prevalent ligand systems. 

The various spectroscopic techniques had revealed that Ti4+ ions in TS-1, Ti-beta and, Ti-MCM-41 are 4-coordinate in the dehydrated state. Tetrapodal Ti(OSi)4 and tripodal Ti(OH)(OSi)3 are the main Ti species. Upon exposure to H20, NH3, H202, or TBHP, they increase their coordination number to 5 or 6. On samples in which the Ti4+ has been grafted onto the silica (referred to as Ti f MCM-41), a dipodal Ti species (Ti(OH)2(OSi)2) may also be present. As a result of interaction with the oxidant ROOH (R = H, alkyl), the formation of 7)1- and p2-peroxo (Ti-O-O-), hydroperoxo (Ti-OOH), and superoxo (Ti02 ) species has been observed experimentally (Section III). A linear correlation between the concentration of the p2-hydroperoxo species and the catalytic activity for propene epoxidation has also been noted from vibration spectroscopy (133). 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

From the relative stabilities of the actinide homoalkyls or -allyls and the tris(cyclopentadienyl) actinide alkyls, it appears that a coordinatively saturated metal center is necessary for kinetic stability. In contrast to f-transition metal alkyls, the absence of hydrogens appears to be of minor importance. In the case of the lanthanide alkyls and the tetrabenzylthorium, where the formal coordination number is only four, the steric bulkiness of the Hgands must be responsible for their observed thermal stability. 

Polymerization in electrostatic systems like the ones mentioned above is stericaUy inhibited by alkyl substitution at the a-carbon which must assume a coordination number greater than 4. Coates and Glockhng have treated this inhibition of polymerization in terms of decreased electronegative character of the branched alkyl groups. Therefore, stimulated by the idea that f-afkylhthium compounds may exist as low polymers or even as monomeric molecules, Weiner and coworkers and Kottke and Stalke have isolated f-butyllithium as a pure substance for the first time and characterised it by spectroscopic methods and X-ray diffraction. The colourless crystalline solid was found to be tetrameric over a range of concentrations in both benzene and hexane ... 

As in the case of the reactions of the coordinated RS group, alkylation results in a weakening of the complexing ability of the ligand as evidenced by an expansion of the coordination number of the nickel ion. Similar reactions have been carried out with the complexes of palladium and platinum, and with all three metals and 2-pyridinaldoxime (POX). In the cases of palladium and platinum, one mole of coordinated ligand tends to be displaced by halide ions (Equation 48). 

The tetradentate ligand forms monomeric square planar complexes. Synthetic and kinetic studies reveal that the coordinated mercapto group may be converted into the coordinated thioether function without breaking the metal-sulfur bond. The nucleophilic power of the coordinated mercapto group exceeds that of RSH, but depends on the metal atom. Bridging protects the sulfur atom from alkylation. In the case of nickel(ll), alkylation is accompanied by expansion of the coordination number of the nickel from 4 to 6. Ligand reactions have led to the synthesis of planar ligands completely cydized about the metal ion. 

Perhaps the best studied group of titanium(IV) complexes is the alkoxides. The metal alkoxides generally have received a great deal of attention because of their ease of hydrolysis and reactivity with hydroxylic molecules, and their tendency to increase the coordination number of the metal which is opposed by the steric effect of the alkyl group. These properties result in materials, the characteristics of which range from polymeric solids to volatile liquids. The definitive review of this area is that by Bradley. ... 

The crux of organic mechanistic stereochemistry may be the Walden inversion, the inversion of stereochemistry about a four-coordinate carbon atom by nucleophilic attack of, for example, a hydroxide ion on an alkyl halide. Many reactions of inorganic molecules follow the same mechanism. In contrast, the dissociative mechanism of tertiary halides to form tertiary carbocatanion intermediates is essentially unknown among the nonmetallic elements silicon, germanium, phosphorus, etc. The reason for this is the generally lower stability of species with coordination numbers of less than 4, together with an increased stability of five-coordinate intermediates. This difference is attributable to the presence of d orbitals in the heavier elements (Chapter 18). 

In this reaction [Mn(CO)5], which is quite nucleophilic, increases its metal coordination number by one. The reaction may be viewed as an electrophilic attack by R+ on the metal. Similarly, it is possible to prepare bridging alkyl complexes by this method/10... 

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