Higher Coordinated Species

Changes from cis- to trans-isomers of some 12-S-6 (C2F4) were reported.63 10.3.4.5 Higher Coordinated Species 

Seven coordinated Te species, such as TeF7-,8 MeOTeF6 ,64a (MeO)2TeF5, 64a (R2NCS2)3TeX,64b and (R2NCS2)3TeR,64c are reported and the structures of 

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The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

The hypervalent chalcogen chemistry has been developed to higher coordinated species with various ligands,7 10 where TBP changes to square pyramidal (SP) or octahedral (Oh), etc. Additional atomic orbitals of E, such as an 5-orbital, may operate to stabilize the structures.10b The concept is also extended over linear a-bonds constructed by m ( > 4) atoms with n electrons (extended hypervalent bonds mc-ne (in > 4)).11 14 The approximate molecular orbital model for mc-ne (m > 4) is also exhibited in Scheme la, exemplified by 4c-6e. 

The direct observation of stable penta(or higher)-coordinate species with eight electrons around the carbon center in solutions was not reported until recent studies of long-lived nonclassical ions in superacid solvent systems. 

In phosphorus ligand complexes coupling of H and 31P often gives useful structural information but in 5 and 7 or higher coordinate species nonrigidity is commonly observed, for example, in Cr(H)2[P(OMe)3]5. 

In general the five- and six-coordinate silicon compounds have chemical shifts at unusually low frequency from — 50 to — 200 ppm, with the six-coordinate compounds appearing at lower frequency from —130 to —200 ppm while the five-coordinate compounds are in the —50 to —150 ppm region. Silicon-29 NMR provides a good method for studying the equilibria between normal four- coordinate silanes and higher-coordinate species in solution as well as characterizing these compounds as penta- or hexacoordinate silicon compounds. 

The IR spectra for chelate treated samples still consist of the two bands that were observed for the original samples but now vary in intensity. The absorbance ratios of the IR bands at 2115 and 2050 cm increase as shown in table 2. These facts suggest that the higher coordinated species is removed form the zeolite by EDTA treatment. To resolve the problem of whether or not effect is due to the chelating ability of EDTA or water solubility, the Fe(CN)-Y(l) and Fe(CN)-Y(2) zeolites were stirred with a NaCl solution. In each case the absorbance ratio of IR bands at 2115 and 2050 cm" increases several times whereas the C and N content decreases (table 2). From these results it appears that a decrease in the intensity of the IR band at 2050 cm" can account for the solubility of higher coordinated species in... 

Each of the compounds 2-4 gave satisfactory microanalysis and El mass and multinuclear NMR spectra. For 2, no 8[ Sn] signal has yet been observed the 5[ Si H ] signals were at 105.02, 9.04 and 7.00. For compound 3 it proved difficult to record the heavier nuclei due to decomposition of the sample. The 6[" Sn H ] and 6[ Si H ] signals for 4 were at -218.1 and 77.05, respectively. Although the value of the " Sn-NMR spectroscopic shift for 4 points to a higher-coordinated species in solution, the X-ray structure shows its divalent nature. Solid-state NMR spectroscopic investigations will be examined. 

As in the case of silica-based sol-gel processes, M—OH groups can also be created by hydrolysis of M—OR groups, that is, by addition of water to metal alkoxides. As outlined above, metal alkoxides are stronger Lewis acids than silicon alkoxides, and the formation of higher coordinated species is easier. Nucleophilic attack at the metal is thus facilitated, and the hydrolysis rates are strongly increased. For example, the hydrolysis rate of Ti(OR)4 is about 10 times faster than that of Si(OR)4 with the same alkoxide substituents. The reactivity of some tetravalent isopropoxides in hydrolysis reactions increases in the following order [4] ... 

In the presence of a nucleophile like a bromide ion, stannyl enol ethers are converted into anionic pentacoordinate stannates, which nucleophilically react with organic halides to give or-alkylated carbonyl compounds. In contrast, such penta-coordinated tin species hardly react with carbonyl electrophiles. The higher coordinated species is also generated by treatment of tin(IV) enol ethers with not only Bu4NBr but an aprotic polar solvent like Such pentacoordinate stannates add in a similar... 

The interaction of species in different oxidation states can lead to higher coordinated molecules (54,116). 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

For all the olefins studied, alkyl-, fluoro-, or chloro-substituted, three binary, mononuclear species were observed. It now seems that it is a general property of Ni and Pd atom-olefin reactions at cryogenic temperatures to form complexes that have a maximum coordination of three olefin molecules per metal atom, regardless of the electronic or steric attributes of the substituent(s). As intimated previously, the absence of higher stoichiometry species, even for unsubstituted ethylene, is, most probably, the result of steric interactions (54). 

In order to make a formal separation between two- and three-center aspects of coordinative bonding, we shall first consider various aspects of simple two-center dative M—L coordination within the framework of normal-valent transition-metal complexes. Aspects of hypervalent cu-bonding to form higher-coordinate complexes (the more common experimental species) will subsequently be considered in Section 4.5.3. 

Figure 5.30 Alternative water pentamer isomers having partial anticooperativity, (a)-(c), or higher coordination and ring strain, (d). Labels in (a)-(c) correspond to clockwise monomer numbering from the top (see the text). (Species (a)-(c) have been optimized under the constraint of planar equilateral skeletal geometry to prevent rearrangement to Wsc [Fig. 5.29(a)] and are therefore only near-stationary points on the potential-energy surface.)...

Niobium and rhodium cluster anions have been prepared by laser vaporization and the reactions with benzene studied by FT-ICR/MS (58). The reactions of the anions and similar cations have been compared. With few exceptions the predominant reaction of the niobium cluster anions and cations was the total dehydrogenation of benzene to form the metal carbide cluster, [Nb C6]-. The Nb19 species, both anion and cation, reacted with benzene to form the coordinated species Nb 9C6I I6p as the predominant product ion. The Nb22 ions also formed some of the addition complex but the Nb2o Nb2i, and all the other higher clusters, formed the carbide ions, Nb C6. ... 

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