Synthesis of the First Examples of Transition Metal .eta.2-SiH4 Complexes, cis-Mo(.eta.2-SiH4)(CO)(R2PC2H4PR2)2, and Evidence for an Unprecedented Tautomeric Equilibrium between an .eta.2-SiH4 Complex and a Hydridosilyl Species: A Model for Methane Coordination and Activation