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

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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.

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