Crystal structure of tetraphenyiarsonium tetranitratozincate(2–), and optical spectrum of tetranitratocobaltate(2–)

Abstract

The title compound crystallises in a monoclinic cell of dimensions: a= 23.355(5), b= 11.334(5), c= 18.588(5)Å, β= 107.5(1)°, space group C2/c. The structure was determined from diffractometer data by the heavy-atom method and refined to R 0.069. Although the eight-co-ordinate [Zn(NO₃)₄]^2– ion has approximately D_2d dodecahedral symmetry, there is a large difference between Zn–O bond lengths r_A and r_B(means 2.06 and 2.58 Å), indicating contributions by a resonance form of the molecule, also reflected in N–O bond lengths Various explanations are given for these bond-length differences, but they probably reflect the tendency of zinc(II) to give tetrahedral co-ordination.

This complex was used es a diamagnetic matrix into which Co²⁺ was dissolved and the polarised optical spectra of the mixed crystals measured. An assignment for [Co(NO₃)₄]^2– in D_2d symmetry is suggested, the sequence of components derived from parent T_d symmetry being: ⁴T₁(F) : ⁴E > ⁴A₂; ⁴T₁(P) : ⁴E > ⁴A₂, ground state ⁴B₁.