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 WSiH 2 + . This ion is itself reactive with a second silane molecule, this time through elimination of 2H 2, to form WSi 2 H 2 + . A similar reaction follows, yielding WSi 3 H 2 + as the next product ion, which itself leads to both WSi 4 H 4 + and WSi 4 H 2 + . This seems to initiate two parallel reaction sequences, yielding WSi 10 H 6 + as the major final product, together with a minor amount of WSi 10 H 4 + . 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 WSiH 2 + and WSi 2 H 2 +, and for the non-observed intermediate WSi 2 H 4 + . 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 WSiH 2 + is a high-spin (sextet) silylene complex in which there is a dative bond between SiH 2 and the metal. For WSi 2 H 4 + 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.