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i_5 0 A . I O h y d r o l y s e d Figure 1. Esterification of chrysotlle-asbestos by allyl alcohol: the units on the ordinate are expressed in milliequivalents of ester groups/100 g of inorganic substrate; degree of hydrolysis reported on the abscissa represents the fraction (5%) of the original magnesium which was eliminated; curve A, hydrolyzed fiber; curve B, chlorinated fiber.out the esterification. The procedure of chlorination is described in detail elsewhere." This reaction substitutes S i c 1 groups for the silanol groups generated by hydrolysis. Results and DiscussionThe results obtained in this study are summarized in Table I1 and in Figure 1. One can see that the degree of esterification increases gradually with the degree of hydrolysis. This result clearly shows that esterification is only possible on the siloxanic surface. One also notices that the degree of coverage is relatively constant and does not depend on the degree of hydrolysis of the fiber. The mean degree of coverage is 316 O R groups/nm2 for the hydrolyzed fibers and 400 OR groups/nmz for the chlorinated ones. This result represents a relative increase of about 26%. This increase in the degree of surface coverage could result from the higher reactivity of the S i 4 groups as compared with that of the silanol groups. If we compare our results with the figure obtained by NotesNotes W a r t m a d for the esterification of silica gel chloride with short chain alcohols, we see that the degree of coverage obtained here is much higher. For example, for the esterification of I-propanol, Wartmann grafted 168 QR/100 nm2. It is our opinion that the higher degree of coverage obtained with chrysotile results from the very regular quasicrystalline structure of the surface produced by hydrolysis compared with the mainly amorphous surface of synthetic silica gel. Iler7 has calculated on a theoretical basis that there are between 800 and 900 OH groups/100 nm* of a silanolic surface. If we take into consideration the problem of steric hindrance introduced by the allylic ester groups, we may expect a maximum degree of substitution of 50%, Le. between 400 and 500 OR groups/ 100 nm2. The experimental values that we have obtained in this study are very close to this theoretical value, especially for the chlorinated fibers.In conclusion, in this study, it has been shown that the reaction of esterification of hydrolyzed or chlorinated chrysotile is possible with an efficiency higher than that obtained for silica gel. This method is very convenient for grafting a continuous layer of organophilic groups onto the surface of phyllosilicates.Acknowledgment. The authors wish to thank Professor J. J. Fripiat, Dr. M. della Faille, and Mr. M. Van Russelt for helpful discussion, They are also indebted to the Institut pour 1'Encouragement a la Recherche Scientifique dans 1'Industrie et 1'Agriculture (I.R.S.I.A.) and to the S. A. Eternit for financial support.Dimeric niobium and tantalum alkoxides have been known for a long time1 and have been the subject of many studies2...

Characterization of 2(0' )8(' 0 )2 by and 13C NMR spectroscopy, IR spectroscopy, and single-crystal X-ray diffraction (-100 °C) is reported. The compound crystallizes in the space group PI with a = 18.262 (4) Á, b = 19.883 (5) Á, c = 12.067 (3) Á, a = 98.59 (1)°, ß = 96.26 (1)°, y = 77.49 (1)°, and Z = 4. The unit cell contains four half-dimers in the asymmetric unit, all of which differ only in the rotational conformation about Zr-O and O-C bonds. In each dimer, the edge-shared bioctahedron has two µ-O'Pr groups. On opposite sides of this 2(µ-)2 plane, each dimer forms two hydrogen bonds, one each between a coordinated alcohol and a terminal alkoxide. The NMR spectra at 25 °C are so simple as to be structurally uninformative, a result of rapid fluxionality which includes, as one component, proton migration among all O'Pr units. At -80 °C in toluene, the NMR spectra are now too complex to be accounted for by a single edge-shared bioctahedral structure. The hafnium analogue is isomorphous with the zirconium compound. Although Ce2(0'Pr)8('Pr0H)2 is not isomorphous, it exhibits an analogous hydrogen-bonded structure in which the 0-0 distance is as short as it is in the Zr analogue, in spite of a metal-metal separation which is longer by 0.28 Á. It crystallizes in the space group PI with a = 1 1.385 (4) Á, b = 12.144 (5) Á, c = 9.009 (3) Á, a = 109.72 (1)°, ß = 111.54 (2)°, y = 65.70 (1)°, and Z = 1. The generality of hydrogen bonding between M-OR and M-0(H)R groups when they are aligned parallel in a metal cluster is reviewed.

Single-components or multicomponent oxide thin films are of interest for electronic and optoelectronic devices, optical applications, catalysis, corrosion protection etc. Their preparation by chemical routes is based on the hydrolytic (sol-gel process) or pyrolytic (MOCVD) conversion of precursors. Derivatives having M-O bonds, namely metal alkoxides, carboxylates or /3-diketonates, are the most common sources of metal oxides. The properties of alkoxides are appropriate for sol-gel as well as MOCVD applications, whilst the limited hydrolytic susceptibility but good volatility of P-diketonates is most convenient for MOCVD purposes. The low temperature and flexibility of sol-gel routes, and the presence of residual OH groups in the final films, are favorable for the encapsulation of organic or organometallic derivatives, the anchoring of enzymes and in general for the development of functional and composite coatings. The facile formation of heterometallic alkoxides is also attractive for the development of coatings based on multimetallic formulations. MOCVD is favorable for the buildup of heterostructures and epitaxial layers. Although metal alkoxides and P-diketonates are usually oxide precursors, nitride or sulfide films can be obtained by reacting with the appropriate reagents. Fluorinated ligands enhance volatility but often result in the formation of metal fluorides. Keywords: Alkoxides, P-diketonates, MOCVD, sol-gel, thin films, oxides, metals, fluorides, sulfides, heterometallic complexes sion protection are well-known examples.' At first, physical methods (sputtering, laser ablation, etc.) were predominant techniques for obtaining films. Chemical methods attract increasing attention, but they often require more elaborate precursors. Thin films can be prepared by spraying, dip-coating or spinning of an appropriate metal precursor solution onto a solid substrate, and are developed mainly for oxide-based materials; solubility of the precursors is a prerequisite. Metal Organic Chemical Vapor-Phase Deposition (MOCVD) techniques concern a larger variety of ceramics, oxides as well as non-oxides, and are better adapted to obtaining epitaxial coatings than are sol-gel techniques, but they need volatile and pyrolyzable precursors.The versatility of chemical routes derives largely from the variety of molecular precursors available as sources. This review sets out to illustrate the potential of precursors having M-0 bonds such as metal alkoxides M(OR),, and pdiketonates, M(OCRCHCR'O), (R, R' = alkyl, aryl, perfluoroalky1)-which can be easily purified by distillation or sublimation-for the formation of oxide thin films, but also fluoride, metal, carbide or oxycarbide, nitride and sulfide coatings, depending o n experimental conditions (temperature, substrate, inert or reactive carrier gas, etc.), and to outline the underlying problems. Metal carboxylates display attractive features for sol-gel applications, especially for the formation of fibers; however, their volatility is generally not relevant for MOCVD and they wi...