Nickel(II) compounds of three homologous linear tetra-amines, especially solvent effects and configuration

Abstract

Abstract A comparative study has been made of the 1:1 nickel(II) compounds of three homologous linear tetraamines: 1,4,7,10-tetraazadecane(trien), 1,4,8,11- tetraazaundecane(2,3,2-tet), and 1,5,8,12-tetraazodecane(3,2,3-tet). A range of solid compounds Ni(tetraamine)X 2 (X = Cl, Br, I, NCS, NO 2 , NO 3 , Clo 4 ), a number of which are new, has been prepared from 2,3,2-tet and 3,2,3-tet. These include the structural isomers yellow low-spin [Ni(3,2,3-tet)]I 2 ] and lilac high-spin trans -[Ni(3,2,3-tet)]I 2 ]. The high-spin species have a trans octahedral configuration, except for [Ni(2,3,2-tet)NO 3 ]NO 3 , which appears to have a chelating nitrate. In solution in water, DMF, and DMSO two equilibria of these compounds have been studied under varying conditions: (a) the cis ⇄ trans equilibria of the high-spin [Ni(N 4 )(S) 2 ] 2+ species (S = solvent); and (b) high spin octahedral ⇄ low-spin planar equilibria. It is shown for the first time that equilibria (a) occur in such solutions for both 2,3,2-tet and 3,2,3-tet. The trans isomers are preferred over cis in the orders: (i) (for solvent) H 2 O > DMF > DMSO; and (ii) (for the amines) 3,2,3-tet > 2,3,2-tet ⪢ trien. In concentrated LiNO 3 and LiClO 4 , Ni(trien) 2+ , but not the others, forms high spin cis -[Ni-(N 4 )O 2 ] species which apparently have a chelating anion. For equilibria (b) (the singlet ⇄ triplet equil.) the planar low-spin species are preferred in the order 2,3,2-tet > 3,2,3-tet > trien — an order different from that observed in (a). Difficulties in defining ϵ max for these low-spin species, and hence in quantifying the equilibria are discussed with evidence for the complication. The observations on both equilibria can be rationalised by reference to the steric effects of the amines: 3,2,3-tet has a greater preference thatn 2,3,2-tet for trans high-spin isomers, but it can less easily accomodate to the shorter NiN bond lengths in the low-spin species.