Vol. 7, Issue 4, Part A (2025)

Spectroscopic absorption and magnetic studies have been used to tune the ligand field in the square planar Ni(diimine) series. Complexes of the type Ni(L)(L'), where L and L' are CH2CH2, CH2CH2CH2, CH2, H2SCH2, or (Ph)2P, exhibit a rich variety of absorption spectra. The spectra of the complexes with L and L' of lower symmetry, such as Ni(Ph2P)2, are generally simple but the positions of the absorption bands vary widely depending on the nature of the diimine ligand. In the complexes of higher symmetry, such as Ni(H2SCH2)2, the spectra are complicated by the presence of ligand hyperconjugation and are difficult to interpret. Values of Dq derived from 10 Dq and the Racah parameters, either by inspection of the positions of the absorption bands or by variable field studies, are useful criteria for judging the ligand field in these complexes. High Dq values, which are likely to be achieved when there is strong pi bonding involving the metal d orbitals, are associated with small Racah B and C values and result in a high spin state. Only the half-filled t2g level is populated in these cases and transitions to the eg level are Laporte forbidden. To assess the ligand field in Ni(II) systems it is also necessary to examine the magnetic properties. High Dq values generally result in the solid state structures being different from that expected for a d8 configuration and the magnetic moments are low due to a large Vt term. Values of Δo derived from magnetic studies often indicate a correct trend in the ligand field but frequently give misleadingly large ligand field strengths. An example of a ligand field stronger than anticipated from a Δo value is Ni(CN)2, which is likely to be an effective pi acceptor and give rise to value of Dq high relative to Δo. In direct contrast with the Ni(diimine) complexes is the square planar Ni(II) phthalocyanine complex, which is a 16 electron species with d8 configuration. This has a ligand field much weaker than that of typical Ni(II) complexes and this is reflected in its high magnetic moment and simple absorption spectrum. A large body of ligand field theory has been developed with the intention of enabling ligand field strengths to be predictively correlated with the spectroscopic and magnetic properties of a complex. This theory is heavily based on molecular orbital methods and does not provide a simple or coherent picture that can be easily related back to more classical crystal field theory. For this reason, it is often difficult to assess the ligand field in particular complexes. In the case of the Ni(diimine) series the experimental data indicates an unusually severe effect of ligand symmetry on the ligand field strength and structure, and in the most complicated cases such as Ni(H2SCH2)2, it may be well-nigh impossible to achieve a conclusive interpretation of the electronic structure.