The magnetic, spectral, and structural properties of trans-tetra-amminedinitronickel(II)

Abstract

The structural (295 and 130 K), magnetic, and spectral properties of trans-[Ni(NH₃)₄(NO₂)₂] are reported. The crystal structure of the compound has been determined by single-crystal X-ray diffraction methods at 295 and 130 K. Crystals are monoclinic, space group C2/m, Z= 2, with a= 1 062.1(4)[1 062.8(4)], b= 681.2(4)[688.7(2)], c= 591.2(3)[597.4(3)] pm, and β= 114.83(3)[114.80(3)]° where the 295 K values are in square parentheses. Diffractometry has provided significant Bragg intensities for 900 (130 K) and 1 264 (295 K) independent reflections and the structures have been refined by full-matrix least-squares methods to R 0.028 (130 K) and 0.051 (295 K). The molecule has a rectangular, almost square [N–Ni–N = 87.5(1)°] plane of ammonia molecules accurately perpendicular to the axis containing the trans-nitrite groups. The molecules are bound together by a fairly simple but strong hydrogen-bonding arrangement. The structure at 130 K is essentially the same as at 295 K. The Ni–N bond lengths, corrected for librational motions, are Ni–N(H₃) 210.9 (211.1) and Ni–N(O₂) 214.2 (215.2) pm where the 295 K values are in parentheses. The magnetic susceptibility follows a Curie–Weiss law with θ=–5.5 K between 10 and 300 K, but departs from this below 10 K to become independent of temperature below 3 K. The data fit the theory for the ground ³A_2g term with zero-field splitting represented by D= 15.2 K(10.4 cm^–1). This zero-field splitting, together with some limited data about the solid-state spectra, is reproduced by a ligand-field treatment with angular-overlap parameters e_σ(NH₃)= 3 700, e_σ(NO₂)= 12 000, e_π(NO₂)= 5 700 cm^–1. The i.r. spectrum, including that of the deuteriated compound, is largely interpreted along conventional lines, but shows a band at 1 850 cm^–1. This band probably arises from strong hydrogen bonding involving one of the ammine hydrogen atoms in particular. This same hydrogen atom is also involved in an anomalously long N–H bond, and has large thermal parameters.