Some studies of Nb₆Cl₁₄,7H₂O and analogous compounds

Abstract

The previously known staphylonuclear halides, Nb₆Cl₁₄,7H₂O; Nb₆Br_l4,7H₂O (in trifling yield) ; Ta₆Cl₁₄,7H₂O ; and Ta₆Br₁₄,7H₂O have been prepared, but intended studies of these have been frustrated by their instability and insolubility. It has been possible, however, to obtain approximate molar conductivities in water and ethanol which support the formulation of (Nb₆Cl₁₂)Cl₂,7H₂O as a 2 : 1 electrolyte, and evidence by pH titration of the existence of the complex ion [(Nb₆Cl₁₂)(OH)₄- (H₂O)_p]^2- in alkaline solution. The absorption spectra of the four halides have been obtained over the range 8-50 kK in neutral and alkaline aqueous solution and in the solid state. It is proposed that the very strong bands in the visible and infrared region result from transitions from metal-to-metal bonding to metal-to-metal anti-bonding levels. Excellent agreement can be obtained between the experimental frequencies of the (Nb₆Cl₁₂) group and those predicted from the molecular orbital energy diagram of Cotton and Haas. It is proposed that Harned's brown chloride is a similar material to the green Nb₆Cl₁₄,7H₂O but additionally contains μ-hydroxo derivatives of the type [Nb₆Cl_(12-n)(OH)_n]Cl₂, resulting from the mode of preparation of the material.