Pentacoordinated, square pyramidal cationic PCP Ni(II) pincer complexes: ELF and QTAIM topological analyses of nickel-triflate interactions

A previous report introduced a new series of cationic nickel(II) complexes ligated by PCP-type pincer ligands featuring a charge-bearing imidazoliophosphine binding moiety and described their catalytic reactivities in hydroamination of nitriles into amidines. Solid-state characterization of the cationic acetonitrile adducts [(R-PIMIOCOP+)Ni(NCMe)(triflate)](+) (R-PIMIOCOP+=(P),(C),(P)-{2-(R2PO),6-(R2PC4H5N2)C6H3}; R=i-Pr, [1](+); Ph, [2](+)) carried out in this follow-up study showed a distorted square pyramidal geometry and a Ni-triflate distance that was shorter than the sum of the Ni and O van der Waals radii, features suggestive of an unusual pentacoordination at the Ni(II) center. In contrast, the related aquo adduct [(i-Pr-PIMIOCOP+)Ni(OH2)(triflate)](+), [3](+), displayed a more conventional square planar geometry. Detailed structural comparisons and theoretical analyses conducted on these and related compounds have allowed a thorough examination of the Ni-triflate interactions in this family of complexes. Thus, topological analysis of the electron localization function (ELF) and quantum theory of atoms in molecules (QTAIM) showed that the Ni-triflate interaction is mostly ionic in nature, but has a weak covalence degree. The monosynaptic V(Ni) subvalence basin of nickel is indeed the ELF signature of the covalence degree of the ionic Ni-O bond, which can be quantified by the negative QTAIM energy density at the Ni-O bond critical point and by the absolute value of the ELF covariance . The ionic character of the Ni-O bond is also reflected in an energy decomposition analysis, showing that this interaction is mostly electrostatic in nature. The computational analyses carried out on this family of complexes provide valuable insight into the character and relative strengths of various Ni-ligand interactions, and allow a number of useful conclusions, including the following: (1) significant Ni-anion interactions at the apical site are observed only with pincer-type ligands featuring at least one cationic imidazoliophosphine binding moiety; (2) these primarily electrostatic Ni-O interactions gain increasing covalence degree when different pincer backbone, co-ligand L, or counter-anions are introduced to enhance the electron deficiency of the Ni(II) center.