Nickel complexes

The Nickel (II) ion forms many stable complexes as predicted by the Irving Williams series. Whilst there are no other important oxidation states to consider, the Ni(II) ion can exist in a wide variety of CN's which complicates its coordination chemistry. For example, for CN=4 both tetrahedral and square planar complexes can be found. For CN=5 both square pyramid and trigonal bipyramid complexes are formed.

The phrase "anomalous nickel" has been used to describe this behavior and the fact that equilibria often exist between these forms. Some examples include:

1. addition of ligands to square planar complexes to give 5 or 6 coordinate species
2. monomer/polymer equilibria
3. square-planar/ tetrahedron equilibria
4. trigonal-bipyramid/ square pyramid equilibria.

(a) substituted acacs react with Ni²⁺ to give green dihydrates (6 coordinate). On heating, the two coordinated water groups are generally removed to give tetrahedral species. The unsubstituted acac complex, Ni(acac)₂ normally exists as a trimer, see below. Lifschitz salts containing substituted 1,2-diaminoethanes can be isolated as either 4 or 6 coordinate species depending on the presence of coordinated solvent.

(b) Ni(acac)₂ is only found to be monomeric at temperatures around 200°C in non-coordinating solvents such as n-decane. 6-coordinate monomeric species are formed at room temperature in solvents such as pyridine, but in the solid state Ni(acac)₂ is a trimer, where each Ni atom is 6-coordinate. Note that Co(acac)₂ actually exists as a tetramer.

[Ni(acac)₂]₃                               [Co(acac)₂]₄

(c) Complexes of the type NiL₂X ₂, where L are phosphines, can give rise to either tetrahedral or square planar complexes. It has been found that:

• L=P(aryl)3 are tetrahedral
• L=P(alkyl)3 are square planar

for L= mixed aryl and alkyl phosphines, both stereochemistries can occur in the same crystalline substance. The energy of activation for conversion of one form to the other has been found to be around 50kJ mol^-1. Similar changes have been observed with variation of the X group:

where Φ is shorthand for C₆H₅. Ni²⁺reacts with CN- to give Ni(CN)₂.nH₂O (blue-green) which on heating at 180-200°C is dehydrated to yield Ni(CN)₂. Reaction with excess KCN gives K₂Ni(CN)₄.H₂O (orange crystals) which can be dehydrated at 100°C. Addition of strong concentrations of KCN produces red solutions of Ni(CN)₅^3-.

The crystal structure of the double salt prepared by addition of Cr(en)₃³⁺to Ni(CN)₅^3- showed that two types of Ni stereochemistry were present in the crystals in approximately equal proportions. 50% as square pyramid and 50% as trigonal bipyramid.