Electrical conductance and ionic association of the bivalent transition metal tetrafluoroborates in acetonitrile solution

Visible absorption spectra of Co(BF₄)₂ and Ni(BF₄)² in acetonitrile (AN) indicate the existence of complex electrolytes solely of the type [M(AN)₆]²⁺·2BF₄^–. Close agreement of the molar conductance curves for Mn(BF₄)₂, Co(BF₄)₂, Ni(BF₄)₂, Cu(BF₄)₂, and Zn(BF₄)₂ indicates that the same is true for the other tetrafluoroborates as well. Small specific differences in properties of the [M(AN)₆]²⁺ complex cations are reflected in the limiting molar conductances, while the first-step association constants to a good approximation are the same for the different metal cations. Penetration of the tetrafluoroborate anion in between the coordinated acetonitrile molecules is suggested as a possible explanation for the apparent independence of ionic association on the crystallographic radius of the cation.