in toluene inert atmosphere; soln. of Nb-complex added dropwise to soln. of AgBF4 (-78°C, stirring), warming to room temp. (30 min., pptn.); collection, washing (toluene), drying (vac.), extn. (CH2Cl2), layering with toluene (crystn.), collection, washing (pentane), drying; elem. anal.;