σ‐Silane Ruthenium Complexes: The Crucial Role of Secondary Interactions

Abstract

The bonding coordination mode of various silanes to ruthenium complexes is discussed on the basis of a detailed analysis of NMR, IR, X‐ray and DFT data. It is now more and more obvious that a continuum exists between the two extreme situations, i.e. formation of a σ‐silane complex or Si–H bond breaking leading to oxidative addition. Within this continuum, secondary interactions involving Si and H play a significant role, in particular for the stabilization of unusual structures or as intermediates in exchange processes. Here, we highlight the criteria allowing the best description of the different bonding modes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

“…The s-H À Si bond length is 1.91(2) by X-ray diffraction (1.891 by DFT/B3LYP calculations) whereas the secondary interaction between the hydride ligand and the silicon atom is characterized by a separation of 1.99(2) by X-ray diffraction (2.076 by DFT/B3LYP calculations). [31] These separations are just on the borderline between primary and secondary Si À H interactions.…”

“…[31] Silane complexes formed the first class of s complexes to be discovered: Hoyano, Elder, and Graham reported the structure of [Re 2 (CO) 8 (m-h 2 ,h 2 -H 2 -SiPh 2 )] in 1969. [32] They described the bonding in a way that is indistinguishable from the modern description: "each silicon-hydrogen bond functions as a two-electron donor to rhenium, effectively taking the place of a carbonyl group; the interaction could be described as a three-center two-electron bond with the two electrons supplied by the original SiÀH bond".…”

“…Similar interactions play a major role in silane activation by ruthenium complexes. [31] This point, which is crucial for the purpose of our Review, will be described in more detail in the next section concerning bis(silane) complexes. In monosilane chemistry, the chlorosilane complex [RuH(h 2 -HSiMe 2 Cl){(h 3 -C 6 H 8 )PCy 2 }(PCy 3 )] is the best representative example.…”

“…[33] Silicon-hydrogen distances in the range of 1.7 to 1.9 (as measured by X-ray crystallography) are normally assigned as s-silane coordination whereas higher values, up to 2.4 , indicate secondary interactions between a silicon atom and an adjacent hydride (see Section 3.2). [31] The [Mo(CO)(diphosphine) 2 ] complexes of H 2 and SiHR 3 allow one of the best comparisons of M(h 2 -Si-H) and M(h 2 -H-H) bonding. The neutron structure of…”

“…The secondary interactions are maximized with the cis geometry of the phosphines, an unusual arrangement for such bulky ligands. [31] The combination of one dihydrogen ligand and one h 2 -SiÀH ligand is illustrated by [Ru(H) 2 [51] The phosphine ligands now adopt a trans arrangement, and the dihydrogen ligand lies parallel to the P-Ru-P axis to avoid competition with the s-borane ligand. The coordination mode of the borane ligand is clearly of a sborane type as attested by a B À H bond length of 1.30(2) (1.362 by DFT/B3LYP), whereas the interaction between the other hydride ligand and the boron atom is shown to be much weaker (separation: 1.89(2) ; 2.003 by DFT/ B3LYP).…”