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 NiL₂Cl₂ 1 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.