This article reviews recent advances in late-transition-metal complexes with chelating Si and Ge ligands. Compounds with two Si-H bonds, such as bis(silyl)alkanes, bis(silyl)arenes, and tetramethyldisiloxanes, react with Re, Rh, Ir, Pd, and Pt complexes to form disilametallacycles with five-or six-membered chelate rings. Four-and five-membered disilacycloalkanes and disilanyldiylcarboranes undergo Si-Si bond cleavage promoted by Pd(0) and Pt(0) complexes to produce the corresponding disilametallacycles. High reactivity of the Si-Si bond toward oxidative addition facilitates the above ring enlargement, even for compounds with stable five-membered rings. Platinum complexes with a η 2 -silene ligand react with small molecules having electronegative atoms, such as O 2 and NH 3 , to produce metallacycles formed via addition of the electronegative atom to the Si-Si bond. The above disilametallacycles undergo insertion of alkynes and carbonyl compounds into the M-Si bonds of the disilametallacycles. The persilyl metallacycle [ À Pt(SiPh 2 SiPh 2 SiPh 2 À SiPh 2 )(PPh 3 ) 2 ] is obtained by the double oxidative addition of tetrakis-(diphenylsilane) with two Si-H groups and via a metathesis reaction of its dilithio derivative with [PtCl 2 (PPh 3 ) 2 ]. Reactions of H 2 GeAr 2 with Pd(II) and Pt(II) complexes having diphenylgermyl ligands yield the tetragerampalladacyclopentane and its Pt analogue. Trigermaplatinacyclobutane reacts with Ph 2 GeH 2 to produce the tetragermaplatinacyclopentane via formal insertion of GePh 2 into a Pt-Ge bond.
C versus Si or Ge MetallacyclesThe chemistry of metallacycles 1 has been a major area of organometallic chemistry because of their role as key intermediates in the metathesis 2 and oligomerization 3 of alkenes catalyzed by transition-metal complexes. Five-and six-membered metallacycles are common, and they are stabilized kinetically in many cases. Their M-C bonds undergo various reactions such as insertion of small molecules, reductive elimination of cycloalkanes, and β-hydrogen elimination, but the reactions occur more slowly than for the usual alkyl complexes due to ring strain of the intermediates or the products. Metallacyclobutanes of Mo, W, and Ru with a four-membered-ring structure are known as the actual intermediates of olefin metathesis catalyzed by the transition-metal complexes. 4 Silyl and germyl complexes of late transition metals exhibit different reactivity from the alkyl metal complexes. For example, they prefer R-hydrogen elimination rather than β-hydrogen elimination, which is common as a decomposition pathway of the alkyl complexes. Chemical properties of the metallacycles, having Si or Ge as the coordinating atoms, are expected to be unique, being cyclic versions of the silyl and germyl complexes. Many research groups have investigated the preparation, structures, and chemical properties of such sila-or germametallacycles. Pd and Pt, in particular, are employed as the major elements among late transition metals, probably because the σ-bonds between Si or Ge and thes...