X-ray structure of thermochromic bis(N,N-diethylethylenediamine)-copper(II)tetrafluoroborate

The structure of a thermochromic complex, bis(N,N-diethylethylenediamine)copper(II)tetrafluoroborate [CuL2(BF4)(2)] having the lowest transition temperature (24 degrees C) amongst its isomorphous counterparts (ClO4- salts of Cu and Ni) has been determined at 22 degrees C thus offering a unique opportunity to study near transition structural parameters. The Cu atom at the crystallographic centre of inversion has square planar coordination from four N-atoms of two centrosymmetrically related ligands [Cu-N(secondary) =2.061(5), Cu-N(primary) = 2.003(6) Angstrom, bite angle N-Cu-N = 84.6(2)degrees], the diethyl group orientations insulate the metal ion from anion coordination [closest approach (Cu ... F3 = 3.621(4) Angstrom)]. The thermal anisotropies of the chelate ring carbons are normal and do not indicate any disorder, although the structure is determined at a temperature (approximate to 22 degrees C) very close to the transition. The highest thermal vibrations are shown by the BF4- anion and then to some extent by the terminal C-atoms of the diethyl groups. Factors involved in the thermochromic phase transition in these systems are discussed.