Synthesis, characterization, and chemical reactivity of zirconium dihydride [(C5H4R)2Zr(.mu.-H)H]2 (R = SiMe3, CMe3). H/D exchange reactions of anionic species [(C5H4R)2ZrH2]-. X-ray crystal structure of [(C5H4SiMe3)2Zr(.mu.-H)H]2