Organolanthanide and organoyttrium hydride chemistry. 9. Bis(1,3-dimethylcyclopentadienyl)yttrium complexes. Synthesis and x-ray crystallographic characterization of (1,3-Me2C5H3)2Y(.alpha.-Me)2, (1,3-Me2C5H3)2Y(.mu.-H)3, and (1,3-Me2C5H3)2(THF)Y(.mu.-H)2