Cycloaddition of CH3CN to a Ph2B(pz) fragment was discovered during explorations of the reactions of the new ligands Ph2B(pz(R,R'))(2)(-) (pz = 3.5-R.R'-pyrazolyl; R = Ph, R' = H and R and R' = CH3) with [Cu(CH3-N)(4)]SbF6. Thus, while treatment of Na[Ph2B(p(2)(Ph,H))(2)] with [Cu(CH3CN)(4)]SbF6 afforded [Ph2B(pz(Ph,H))(2)]Cu-(Hpz(Ph,H)) that exhibited the expected bidentate coordination of the bis(pyrazolyl)borate ligand in its X-ray crystal structure, the analogous reaction with Na[Ph2B-(pz(3)(3)(CH)(CH))(2)] led to the product shown in the figure.