in tetrahydrofuran addn. of HfCl4 at 0°C to freshly prepared C5H4C2H5Li in THF withstirring over 0.5 h, mixture stirred for further 24 h at 20°C; THF removed under vac., residue extd. with boiling toluene and little activated charcoal, filtered, addn. of hexane to filtrate, cooling to -30°C, elem. anal.;