Twenty-three diarylcarbenium ions and 38 pi-systems (arenes, alkenes, allyl silanes and stannanes, silyl enol ethers, silyl ketene acetals, and enamines) have been defined as basis sets for establishing general reactivity scales for electrophiles and nucleophiles. The rate constants of 209 combinations of these benzhydrylium ions and pi-nucleophiles, 85 of which are first presented in this article, have been subjected to a correlation analysis to determine the electrophilicity parameters E and the nucleophilicity parameters N and s as defined by the equation log k(20 degrees C) = s(N + E) (Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957). Though the reactivity scales thus obtained cover more than 16 orders of magnitude, the individual rate constants are reproduced with a standard deviation of a factor of 1.19 (Table 1). It is shown that the reactivity parameters thus derived from the reactions of diarylcarbenium ions with pi-nucleophiles (Figure 3) are also suitable for characterizing the nucleophilic reactivities of alkynes, metal-pi-complexes, and hydride donors (Table 2) and for characterizing the electrophilic reactivities of heterosubstituted and metal-coordinated carbenium ions (Table 3). The reactivity parameters in Figure 3 are, therefore, recommended for the characterization of any new electrophiles and nucleophiles in the reactivity range covered. The linear correlation between the electrophilicity parameters E of benzhydryl cations and the corresponding substituent constants sigma(+) provides Hammett sigma(+) constants for 10 substituents from -1.19 to -2.11, i.e., in a range with only very few previous entries.

The synthesis of trimethylsilyl-substituted poly(titaniumcarbodiimide) as a novel precursor for titanium carbonitride based ceramic materials is described. The precursor and the subsequent processing steps (cross-linking and pyrolysis) are characterized by IR and Raman spectroscopy, thermal gravimetric analysis and simultaneous mass spectroscopy, electron microscopy and powder X-ray diffraction measurements. The novel polymer is formed by the reaction of TiCl 4 or Ti(NEt 2 ) 4 with bis(trimethylsilyl)carbodiimide. Subsequent pyrolysis at 1000°C in argon results in the formation of a ceramic composite material consisting of nanocrystalline TiCN and amorphous SiCN as constituting phases. Using Ti(NEt 2 ) 4 as a starting reagent instead of TiCl 4 , chlorine contamination of the ceramic material can be avoided. The different molecular vibration modes of the metal-nitrogen, metal-carbon and nitrogencarbon bonds in poly(titaniumcarbodiimides) with trimethylsilyl substituents were calculated using quantum mechanical methods, providing a comprehensive understanding of the measured spectra.