Group 9 elements as donors of hole bonds.

This study presents experimental and theoretical evidence supporting the existence of net attractive noncovalent interactions between group 9 elements (Co, Rh, and Ir) and lone-pair-possessing atoms (nucleophiles and Lewis bases). This work expands the scope of σ-/π-hole interactions to include group 9 metals. Analyses of the Cambridge Structural Database (CSD) reveal that porphyrin derivatives of Co(II), Co(III), Rh(III), and Ir(III) as well as their phthalocyanine and corrole analogues exhibit contacts with nucleophiles that are too long to be rationalized as coordinative covalent bonds. To rationalize these interactions, Density Functional Theory (DFT) calculations were performed on model porphyrin complexes of Co(II) and M(III) (M = Co, Rh, and Ir) interacting with simple Lewis bases. Molecular Electrostatic Potential (MEP) surfaces of the metal porphyrin confirm the presence of regions of depleted electron density corresponding to σ-holes on the metal, thus enabling these atoms to serve as electrophiles. The attractive nature and distinctive features of these noncovalent contacts are further substantiated by detailed computational analyses: interaction energy (Eint) calculations; Energy Decomposition Analysis (EDA); Natural Bond Orbital (NBO) analysis; and Quantum Theory of Atoms in Molecules (QTAIM) analysis. We propose the term nonagen bond (NnB) to denote these newly characterized noncovalent interactions involving group 9 elements as the electrophilic atom.