Variable geometry of nickel(II) complexes with N1-isopropyl-2-methyl-1,2-propanediamine(L). Crystal structures of [NiL2]Br2·3H2O, [NiL2Br]Br and [NiL2(NCS)2] and solid-state thermochromism in [NiL2]Br2·3H2O

Abstract

The complexes NiL₂X₂·nH₂O (L =N¹-isopropyl-2-methylpropane-1,2-diamine; n= O, X = Cl, NCS or Br; n= 3, X = Br) have been prepared and the crystal structures of [NiL₂]Br₂·3H₂O 2, [NiL₂Br]Br 3 and [NiL₂(NCS)₂]4 determined. The metal co-ordination in complex 2 is square planar (N₄ chromophore), whereas in 3 it is a distorted trigonal bipyramid with one of the bromine atoms and the two primary nitrogen atoms defining the basal plane, the secondary nitrogen atoms being in axial positions. Complex 4 is trans octahedral with a N₆ chromophore. The geometry of NiL₂Cl₂1 is proposed to be five-co-ordinate as in 3. Complex 2 on heating, undergoes an irreversible exothermic isomerisation after dehydration transforming into 3, a unique example of a change from square-planar to trigonal-bipyramidal nickel(II), accompanied by a yellow to green thermochromism in the solid state.