Crystal and molecular structure of aquobis(ethylenediamine)(tetrafluoroborato)nickel(II) tetrafluoroborate. A cis-octahedral nickel(II) complex with a co-ordinated tetrafluoroborate ion

Abstract

Both an aquo-[Ni(en)₂(H₂O)(BF₄)](BF₄)(I), and a bisaquo-complex (II)[Ni(en)₂(H₂O)₂](BF₄)₂, may be obtained from aqueous solutions. The electronic spectra and easy loss of water from (II) suggest a trans-octahedral structure, and similar data for (I) indicate the presence of a cis-octahedral structure, and hence of a co-ordinated BF₄^– ion. This has been confirmed by a crystal structure analysis. Crystals of (I) are orthorhombic, space group P2₁2₁2₁ with a= 7·831(10), b= 13·118(10), c= 13·541(10)Å, and Z= 4. The structure was solved by use of three-dimensional X-ray diffraction methods from visually estimated data and refined by leastsquares techniques to R 8·2% for 1267 independent non-zero reflections. The ethylenediamine ligands are in a cis-arrangement about the metal ion and are in a gauche configuration. The remaining two octahedral positions are occupied by a water molecule and a fluorine atom of a BF₄^– ion, with Ni–F 2·12 Å. There is strong hydrogen bonding between the hydrogens of the water molecule, a fluorine atom from a bonded tetrafluoroborate, and a further fluorine atom from a nonbonded BF₄^– ion. Solid-state electronic spectra of (I) show that BF₄^– lies slightly higher than H₂O in the spectrochemical series. Complex (II) in solution shows an electronic spectrum attributable to a cis-[Ni(en)₂(H₂O)₂]²⁺ species, whilst (I) gives a spectrum which appears to be more characteristic of a cis-[NiN₂O₄] chromophore than of [NiN₄O₂]. This suggests that, in the latter, there is a labilizing effect of the two atoms (O and F) on the two nitrogen atoms of the ethylendiamine trans to them.