NovelMetal-to-MetalSilyl-MigrationReactionsinHeterometallicComplexes Pierre Braunstein,* [a] MichaelKnorr, [a, d] Georg Reinhard, [b] Ulrich Schubert, [b, c] and Thomas Stährfeldt [a] Dedicated to Professor J. Amaudrut, on the occasion of his 60th birthday Abstract: An unprecedented, intramo- lecular metal-to-metal silyl ligand mi- gration reaction has been discovered in a series of phosphido-bridged iron-plati- num complexes and which may be triggered by an external nucleophile. Thus, reaction of solutions of [OC) 3 - R 1 3 Si)F e m -PR 2 R 3 )P t1,5-COD)] 1a R 1  OMe, R 2  R 3  Ph; 1b R 1  OMe, R 2  R 3  Cy; 1c R 1  Ph, R 2  R 3  Ph; 1d R 1  Ph, R 2  R 3  Cy; 1e R 1  Ph, R 2  H, R 3  Ph) in CH 2 Cl 2 with CO rapidly afforded the corresponding com- plexes [OC) 4 F e m -PR 2 R 3 )P tSiR 1 3 - CO)] 2a ± e) in which the silyl ligand has migrated from Fe to Pt, while two CO ligands have been ligated, one on each metal. When 1a or 1c was slowly treated with two equivalents of tBuNC at low temperature, quantitative dis- placement of the COD ligand was ac- compagnied by silyl migration from Fe to Pt and coordination of an isonitrile ligand to Fe and to Pt to give [OC) 3 - tBuNC)F e m -PPh 2 )P t{SiOMe) 3 }- CNtBu)] 3a) and [OC) 3 tBuNC)- F e m -PPh 2 )P t{SiPh 3 }CNtBu)] 3c). Re- action of 2a with one equivalent of tBuNC selectively led to substitution of the Pt-bound CO to give [OC) 4 - F e m -PCy 2 )P t{SiOMe) 3 }CNtBu)] 4b), which reacted with a second equivalent of tBuNC to give [OC) 4 Fem-PCy 2 )- Pt{SiOMe) 3 }CNtBu) 2 ]5b) in which the metal ± metal bond has been cleaved. Opening of the Fe Pt bond was also observed upon reaction of 3a with tBuNC to give [OC) 3 tBuNC)- Fem-PPh 2 )Pt{SiOMe) 3 }CNtBu) 2 ]6). The silyl ligand migrates from Fe, in which it is trans to m-PR 2 R 3 in all the metal ± metal-bonded complexes, to a position cis to the phosphido bridge on Pt. However, in 5a,b and 6 with no metal ± metal bond, the Pt-bound silyl ligand is trans to the phosphido bridge. The intramolecular nature of the silyl migration, which may be formally viewed as a redox reaction, was estab- lished by a cross-over experiment con- sisting of the reaction of 1a and 1d with CO; this yielded exclusively 2a and 2d. The course of the silyl-migration reac- tion was found to depend a) on the steric properties of the  SiR 1 3 ligand, and for a given m-PR 2 R 3 bridge R 2  R 3  Ph), the migration rate decreases in the sequence SiOMe) 3 > SiMe 2 Ph > SiMePh 2  SiPh 3 ; b) on the phosphido bridge and for a given silyl ligand R 1  OMe), the migration rate decreases in the order m-PPh 2  m-PHCy; c) on the external nucleophile since reaction of 1c with two equivalents of POMe) 3 , POPh) 3 or Ph 2 PCH 2 CO)Ph led solely to displacement of the COD ligand with formation of 11a ± c, respectively, whereas reaction with two equivalents of tBuNC gave the product of silyl migration 3c. Reaction of [OC) 3 - {MeO) 3 Si}F e m -PPh 2 )P tPPh 3 ) 2 ] 7a) with tBuNC even in slight excess) occurred stereoselectively with replace- ment of the PPh 3 ligand trans to m-PPh 2 , whereas reaction with CO led first to [OC) 3 {MeO) 3 Si}F e m -PPh 2 )P tCO)- PPh 3 )] 8a), which then isomerized to the migration product [OC) 4 F e m - PPh 2 )P t{SiOMe) 3 }PPh 3 )] 9a). Most complexes were characterized by ele- mental analysis, IR and 1 H, 31 P, 13 C, and 29 Si NMR spectroscopy, and in five cases by X-ray diffraction. Keywords: bimetallic complexes ´ iron ´ P ligands ´ platinum ´ Si ligands ´ silyl migration Introduction The increasing interest in organosilicon chemistry has been triggered by recent synthetic and mechanistic discoveries of fundamental importance and by advanced applications. [1±3] A rich and versatile chemistry is not unexpected for an element like silicon, which can be surrounded in its compounds by a number of neighbors ranging from one e.g., Si  O) to ten e.g., SiC 5 Me 5 ) 2 ). Synthetic and reactivity studies on metal ± silicon bonds have become a major research area in organo- metallic chemistry and although the first transition metal silyl complex [CpOC) 2 Fe-SiMe 3 ] was reported more than 40 years ago, [4] the nature of the metal ± silicon bond is still the subject of thermodynamic, spectroscopic, and theoretical studies. [1±3] In this context, it is interesting that heterobimetallic silicon chemistry has only recently become a well-identified research [a] Dr. P. Braunstein, Dr. M. Knorr, T. Stährfeldt Laboratoire de Chimie de Coordination UMR 7513 CNRS) Institut Le Bel, Universite  Louis Pasteur 4 rue Blaise Pascal, 67070 Strasbourg France) Fax:  33)3-88-41-60-30 E-mail: braunst@chimie.u-strasbg.fr [b] G. Reinhard, Prof. Dr. U. Schubert Institut für Anorganische Chemie der Universität Würzburg Am Hubland, 97074 Würzburg Germany) [c] Prof. Dr. U. Schubert Institut für Anorganische Chemie der Technischen Universität Wien Getreidemarkt 9, 1060 Wien Austria) [d] Dr. M. Knorr New address: Laboratoire de Chimie et Electrochimie Mole Âculaire Faculte  des Sciences et Techniques Universite  de Franche-Comte  16 Route de Gray, 25030 Besançon France) E-mail: michael.knorr@univ-fcomte.fr FULL PAPER Chem. Eur. J. 2000, 6, No. 23  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0947-6539/00/0623-4265 $ 17.50+.50/0 4265