Redistribution at silicon by ruthenium complexes. Bonding mode of the bridging silanes in Ru 2 H 4 (μ-η 2 :η 2 :η 2 :η 2 -SiH 4 )(PCy 3 ) 4 and Ru 2 H 2 (μ-η 2 :η 2 -H 2 Si(OMe) 2 ) 3 (PCy 3 ) 2