d–d Spectral assignments in tetragonal nickel(II) complexes

Abstract

Cellular ligand field (CLF) analyses have been made for seven tetragonal-octahedral nickel(II) complexes [NiCl₂(tfeh)₄][tfeh =(tetrafluoroethyl)hydrazine] and [NiX₂([n]aneN₄)](X = Cl^– or NCS^–; n= 14, 15 or 16 and [n]aneN₄ represents an n-membered tetraaza macrocycle). Published assignments of the d–d spectra for all bar the [14]aneN₄ complexes lead to a π-acceptor function for Cl^– and NCS^–. This result is chemically unreasonable for Cl^– and previous studies indicate that NCS^– is not a π acceptor towards Ni^II either. The d–d spectra have been reinterpreted within the CLF framework leading to a consistent set of assignments and ligand-field parameters. The latter monitor the interplay between axial and equatorial bonding consistent with Pauling's electroneutrality principle: i.e. as the strength of the equatorial donation decreases [e_σ(N) gets smaller] the axial donors bind more strongly to compensate [e_σ(X) and e_π(X) increase] such that the sum, Σ, of all the CLF parameters remains essentially constant for the same donor sets. For the macrocyclic complexes the significant increase in the axial bonding parameters need not necessarily be accompanied by a marked decrease in the Ni–X bond length. Steric interactions between H atoms on the macrocyclic ring and the axial donors appear to prevent a closer approach.