The dehydrocoupling of the sterically hindered phosphine-borane adduct tBu(2)PH.BH(3) above 140 degrees C is catalyzed by the rhodium complexes [Rh(1,5-cod)(2)][OTf] or Rh(6)(CO)(16) to give the four-membered chain tBu(2)PH-BH(2)-tBu(2)P-BH(3) (1), which was isolated in 60% yield and characterized by multinuclear NMR spectroscopy, mass spectrometry, and elemental analysis. Thermolysis of 1 in the temperature range 175-180 degrees C led to partial decomposition and the formation of tBu(2)PH.BH(3). When the dehydrocoupling of tBu(2)PH.BH(3) was performed in the presence of [[Rh(mu-Cl)(1,5-cod)](2)] or RhCl(3) hydrate, the chlorinated compound tBu(2)PH-BH(2)-tBu(2)P-BH(2)Cl (2) was formed which could not be obtained free of 1. The molecular structures of tBu(2)PH.BH(3), tBu(2)PH-BH(2)-tBu(2)P-BH(3) (1), and tBu(2)PH-BH(2)-tBu(2)P-BH(2)Cl (2) together with 1 were determined by single-crystal X-ray diffraction studies.

The first hypercoordinate silicon-bridged [1]ferrocenophane shows a pronounced Si-N interaction, a pseudo-trigonal-bipyramidal geometry at the bridging silicon atom, an elongated Si-Cp bond trans to the amino group, and an unusually long Fe-Si distance in comparison to the tetracoordinate analogues.

The reaction of Ph2P(O)(OSiMe3) with [(η5-C5H5)TiCl3] or [(η5-C5Me5)TaCl4] in dichloromethane yields the complexes [{(η5-C5H5)TiCl2(µ-Ph2PO2)}2] (1) or [{(η5-C5Me5)TaCl3(µ-Ph2PO2)}2] (2), respectively. Structural analysis of 1 and 2 by single-crystal X-ray diffraction revealed the presence of eight-membered metallacycles consisting of bridging–bidentate diphenylphosphinate groups between two metal centers.Key words: inorganic rings, phosphinate complexes, tantalum, titanium.

(1 d), respectively). Single-crystal X-ray diffraction studies revealed pseudotrigonal bipyramidal structures for both 5 a and 5 b, with one of the shortest reported Si ¥¥¥ N distances for an sp 3 -hybridized nitrogen atom interacting with a tetraorganosilane detected for 5 a (2.776(2) ä). Elongated Si À C ipso bonds trans to the donating NMe 2 arms (1.919(2) and 1.909(2) ä for 5 a and 5 b, respectively) were observed relative to both the non-trans bonds (5 a: 1.891(2); 5 b: 1.879 (2)