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

The title compound crystallises in a monoclinic cell of dimensions: a= 23.355(5), b= 11.334(5), c= 18.588(5)A, 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 D2d dodecahedral symmetry, there is a large difference between Zn–O bond lengths rA and rB(means 2.06 and 2.58 A), 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 D2d symmetry is suggested, the sequence of components derived from parent Td symmetry being: 4T1(F) : 4E > 4A2; 4T1(P) : 4E > 4A2, ground state 4B1.