Preparation and properties of the heptacyanomolybdate(II) anion and the crystal and molecular structures of its sodium and potassium salts

Abstract

The compounds M^I₅[Mo(CN)₇]·xH2O [(II), M^I= Na, x= 10; (I) M^I= K, x= 1] have been isolated from the reaction of molybdate(VI) with cyanide and hydrogen sulphide in aqueous solution and by reduction of the [Mo(CN)₄(O)₂]^4– ion with hydrazine. Crystals of (I) are triclinic, Z= 2, space group I1, a= 9.069(8), b= 9.218(8), c= 9.029(8)Å, α= 89.51(8), β= 90.17(9), γ= 92.47(8)°. Crystals of (II) are triclinic, Z= 2, space group P, a= 16.527(12), b= 8.355(8), c= 8.324(9)Å, α= 105.48(9), β= 80.90(9), γ= 107.43(9)°. 1 175 (K) and 2 790 (Na) independent reflections were measured on a diffractometer for the two compounds; structures were solved by the heavy-atom method and refined by full-matrix least squares to R 0.081 and 0.066 respectively.

In both salts the geometry of the heptacyanide ion is best described as a pentagonal bipyramid, but while in the Na salt the symmetry is D_5h, in the K salt the ion is much more distorted due to packing effects, one CN group being displaced (C 1.03, N.1.54 Å) from the MoC₄ plane giving a C₈ structure. I.r. and u.v.–visible spectra and magnetic properties are reported and discussed.