Organolanthanide and organoyttrium hydride chemistry. 10. Reactivity of trimetallic organoyttrium hydride complexes. Synthesis of the alkoxy hydride anions [[(C5H5)2Y(.mu.-H)]x[(C5H5)2Y(.mu.-OCH3)]3-x(.mu.3-H)]- (x = 0-2), including the x-ray crystal structure of [[(C5H5)2Y(.mu.-OCH3)]3(.mu.3-H)]2[Li(THF)3]2