in toluene (N2); addn. of a soln. of tin complex in toluene to a suspn. of niobium complex in toluene at room temp., stirring for 4 d in the dark; evapn., addn. of CH2Cl2, stirring for 2 d, filtration, concn., addn. of hexane, crystn. at -25°C, washing with Et2O, drying in vac.; elem. anal.;