Activation of Silane by W+ in the Gas Phase: Fourier-Transform Ion Cyclotron Resonance and ab Initio Theoretical Studies

The gas-phase reactions of the bare tungsten cation W + with silane have been investigated using Fourier- Transform Ion Cyclotron Resonance mass spectrometry. Dehydrogenation of a first molecule leads to the formation of WSiH2 + . This ion is itself reactive with a second silane molecule, this time through elimination of 2H 2, to form WSi2H2 + . A similar reaction follows, yielding WSi3H2 + as the next product ion, which itself leads to both WSi4H4 + and WSi4H2 + . This seems to initiate two parallel reaction sequences, yielding WSi 10H6 + as the major final product, together with a minor amount of WSi10H4 + . CID experiments on the products of the first three reactions were carried out to aid in their structural elucidation. Ab initio calculations at the CASSCF level have been performed in order to derive optimum structures for the first two product ions WSiH2 + and WSi2H2 + , and for the non-observed intermediate WSi2H4 + . The results show that structural isomerism exists for these three ions, due to the versatile bonding capabilities of W + and Si. The ground state of WSiH2 + is a high-spin (sextet) silylene complex in which there is a dative bond between SiH2 and the metal. For WSi2H4 + there are two low-energy isomers, a covalently bonded metal disilene three-membered ring with a quartet spin state, and a datively bonded metal silylsilylene in a high-spin sextet. It is proposed that the successive ions formed have compact structures, and that the reaction sequence ends when the metal gets trapped into a silicon cage, or alternatively when it no longer has enough nonbonding electrons to insert exothermically into a Si-H bond of a further silane molecule.