The role of siloxide ligands in the chemistry and catalysis of transition metals is reviewed. The first part deals with recent aspects of low-valent transition metal siloxide chemistry, while the second part describes some developments in the chemistry of transition metal siloxides in high oxidation states, in particular with regard to the stability of the M-OSi

The synthesis, structural characterization, and acid-base chemistry of [C(SiMe2OCH2CH2OMe)3]Na (2), a sterically encumbered zwitterionic organosodium compound, is reported. 2 is a strong Brønsted base that forms frustrated Lewis pairs (FLPs) with a number of boron-containing Lewis acids ranging from weakly Lewis acidic aryl and alkyl boranes to various alkyl borates. These intermolecular FLPs readily cleave H2, which confirms that even poor Lewis acids can engage in FLP-mediated H2 cleavage provided that the present bulky base is of sufficiently high Brønsted basicity.

A number of new oligosilane chains containing linear segments with up to 18 silicon atoms were prepared. Employing salt elimination reactions of oligosilylpotassium compounds and oligosilylhalides, we prepared a number of tris(trimethylsilyl)silyl-terminated chains with varying internal segments consisting of dimethylsilylene and bis(trimethylsilyl)silylene units. Single-crystal X-ray diffraction and UV spectroscopic analysis indicate that the oligosilanes thus obtained are conformationally highly constrained due to steric interactions between trimethylsilyl groups. The chains consist of all-transoid-oriented segments, capable of σ-bond electron delocalization, which are separated by cisoid turns.

A new approach to intermolecular frustrated Lewis pairs (FLPs) that combines readily available bulky organosuperbases with moderate to weak boron-containing Lewis acids is reported. These so-called "inverse" FLPs are demonstrated to heterolytically cleave dihydrogen, allowing for the isolation and structural characterization of various phosphonium borohydride salts; two FLPs proved to be efficient catalyst's in the metal-free hydrogenation of N-benzylidenaniline.