A Search by NBO Analysis for Syndetic Donation in Known X‐ray Structures of Transition Metal Complexes with Close Aromatic Ring C–H···Metal Approaches

Abstract

Natural bond orbital (NBO) analysis obtained from density functional theory (DFT) calculations shows that where close approaches of aromatic ring C–H bond hydrogens to transition metal centers indicate an agostic interaction might be present, the angle the ring makes with the coordination plane and the metal···Cagostic separation can determine how significant are the agostic, syndetic (π or σ‐electron density in the vicinity of the agostic carbon interacting with a metal center) and back‐donations that are present. The study reveals that in the few X‐ray crystal structures that are known where agostic bonding is present, the interaction is weak, but the presence of syndetic donation is confirmed in the solid state but is small in magnitude. Agostic interactions present in a Co‐aminophosphane, and Ir and Ru 2‐phenylpyridine complexes show syndetic π‐donation components with NBO E(2) values of only 3.2, 2.1 and 2.1 kcal mol–1 respectively whereas a Rh tris‐triphenylphosphine cation shows minimal agostic donation and η2‐π‐donation and a Pd complex with two dicyclohexylbiphenylphosphine ligands has minimal agostic or syndetic components and appears to contain unusual aromatic ring rotations between the anagostic and agostic states. The work reinforces the multicomponent nature of agostic interactions involving aromatic rings.