in ethanol inert atmosphere; refluxing (48 h); filtration (in air), cooling to room temp. (crystn.), filtration, washing (cold EtOH, Et2O), recrystn. (hot acetone or hot EtOH); elem. anal.;

in dichloromethane stirred for 4 d at room temp.; concd. (vac.), toluene added, filtered after 1 h, crystals washed with toluene, dried (vac.);

sodium cyclopentadienide in benzene 25°C, 2 days;;

20°C;;

in acetonitrile addition of CpSnBu3 in acetonitrile to NbCl4 at 0°C, stirred for3 h, evapn., dissolving in THF, CpLi addition, stirred for 0.5 h, inertatm.; evapn. in vac., washing with heptane, sublimation (200-250°C, 6E-4 torr); elem. anal.;