Hexacoordinate Monomers

The electronic absorption spectra of the hexacoordinate complexes M(CNPh)6 (M = Cr, Mo, W) each show three or four absorption bands between 

250 nm and 500 nm due to dn- n (MLCT) transitions. These bands are at lower energy than those found in the M(CO)6 (M = Cr, Mo, W) series because of stabilization of the n orbital due to conjugation with the phenyl ring of the isocyanide ligand, and also because of the increased electron density on the metal center due to the higher basicity of PhNC as compared to 

The analogous hexacoordinate complex Fe(CNMe)6 also undergoes photosubstitution reactions. Photolysis of this complex at 254 nm or 365 nm in the presence of water gives the aquo complex Fe(CNMe)4(H20)2, presumably via the intermediate formation of the monosubstituted aquo complex Fe(CNMe)5(H20) (Ref. 212)  

A similar reaction between water and the mixed ligand complex Fe(CN)2(CNMe)4 results in photosubstitution of the methyl isocyanide ligand to give first Fe(CN)2(CNMe)3(H20), then the diaquo complex Fe(CN)2(CNMe)2(H20)2 as the final product. 2  

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A completely different model of the origin of the syndiospecificity which involves the formation of a lluxional chiral site has also been proposed.147 According to this mechanism, the chirality of the growing chain determines the chirality of the fluxional site, which in turn discriminates between the two monomer enantiofaces. In particular, the assumed model site consists of a hexacoordinated metal (V) atom surrounded by four chlorine atoms assumed to be bridge bonded to other metal (i.e., Al) atoms147 149 (Figures 22a,b). 

OrganometaUic compounds, of transition metals as catalysts for polymerization of vinyl monomers and olefins, 23 263-325 OrganometaUic transformation, molecular analogs, 38 288, 290-291 Organophosphorus substrates, hydrolysis of, effect of cycloamyloses on, 23 235 Organosilicon compounds, hexacoordinated, 34 155... 

The synthetic route (c) was recently utilized to prepare unsaturated monomers containing the hexacoordinate silicon unit, followed by polymerization to form novel polymers with hexacoordinate silicon in the polymer chain (equation 49)214. The29Si chemical shift measured in the polymer solution (ca —60 ppm) is very similar to that of the monomer (—63.8 ppm), and is evidence for hexacoordination in the polymer solution. 

According to most models proposed for polymerisation with vanadium-based homogeneous Ziegier-Natta catalysts such as VCI4—A1R2C1, the active site may involve pentacoordinated three-valent vanadium species with three Cl atoms, the secondary carbon atom of the last monomer unit inserted (predominantly by 2,1 enchainment of the a-olefin) and the coordinating olefin [323-327]. However, a model of the active site that assumes a hexacoordinated V(III) species with four Cl atoms, the carbon atom of the last chain unit and the coordinating olefin has also been proposed [328],... 

To suppress the formation of self-complementary monomers and their polymerization by making the external pockets of the cryptates too small to host the external cesium ions, H2L5 (13) with bulkier phenyl groups was used. To that end, stoichiometric amounts of H2L5, alkali acetates, and divalent hexacoordinate metal acetates were reacted (Scheme 6). Since the polymers meso-14 are isostructural, only the structure of meso- 14e is discussed in detail. In meso- 14e, two 2 -metallocryptate... 

On the other hand, diethyl l,4-butanediylbis(aminomethylene)-bis(cyanoacetate) H2(L24) (61) reacts with copper(II) acetate to give coordination polymer J [Cu(L24)J (62). In contrast to the hexacoordinate examples discussed so far, in 62 copper is only pentacoordinate. This leaves one cyano group of monomer [Cu(L24)] (63) unoccupied and as in stair-like 59 leads to reduction of dimensionality resulting in a zig-zag lD-stmcture for 62 (Scheme 23, Fig. 23) [168]. 

The key features of this mechanism are the activation of the silyl ketene acetal initiator by the nucleophilic catalyst, the further coordination of the monomer to the nucleophilic silicon centre, rendering the silicon hexacoordinate, and the transfer of the silyl group to the incorporated monomer, thus regenerating the reactive silicon group to continue the polymerization. 

The above model shows the insertion of monomers on a-metal bonds [239, 240]. NMR spectroscopy supports the hypothesis [243]. Also, the model shows the vanadium to be penta-coordinated, following a Zambelli and Allegra suggestion [243]. Hexacoordinated vanadium has... 

Ru and Fe complexes in which two bipy ligands saturate the hexacoordination at PVP or PVbipy chains are among the most conmuMi residues they were also obtained on the electrode stuface by direct cathodic reduction of the relevant monomer, as reported in the already cited Ref. [8]. 

Polymerization of CL exhibited approximately first-order propagation both in monomer (internally) and in active species. The apparent fep value (encompassing both five- and six-coordinate species) is equal to 0.621mor s" (25 °C, THF). Most probably, propagation proceeds on the more labile five-coordinate species in intramolecular dynamic equilibrium with the hexacoordinate one. Thus, the absolute fep could be considerably higher. 

MoBbauer spectroscopy is another important method to obtain additional information about the structure of the bridged phthalocyaninatoiron compounds. The isomer shift (6) and the quadrupol splitting (AE ) have been measured for the monomer (6 = 0.11 mm/s, AE = 0.65 mm/a). The data of [PcFe(Me dib)] 6 = 0.14 mm/s and AE = O.o9 mm/s are another definite proof for the hexacoordination of tne dib-bridged compounds. 

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