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)]2Li(THF)32