π Acceptor Abilities of Anionic Ligands: Comparisons Involving Anionic Ligands Incorporated into Linear d¹⁰ [(NH₃)Pd(A)]⁻, Square Planar d⁸ [(NN₂)Ru(A)]⁻, and Octahedral d⁶ [(AsN₄)Tc(A)]⁻ Complexes

Extended transition state-natural orbitals for chemical valence (ETS-NOCV)] data were used to rank electron acceptor capacities for several potentially synergistic anionic ligands incorporated into linear d¹⁰ [(NH₃)Pd(A)]⁻, square planar d⁸ [(NN₂)Ru(A)]⁻, and octahedral d⁶ [(AsN₄)Tc(A)]⁻ complexes [A = anionic ligand, NN₂ = HN(CH₂CH₂CH₂NH₂)₂, and AsN₄ = [As(CH₂CH₂CH₂NH₂)₄]⁻]. It was possible to differentiate between the best acceptors, among them BI₂^– and B(CF₃)₂^–, and the poorest ones. A sizable fraction of the anionic ligands studied exhibit similar acceptor capacities (backbonding), mostly regardless of d electron count. A number of trends were discerned, including the fact that acceptor capacity decreases down families and across rows but increases down families of the peripheral substituents. The latter appears tied to the ability of the peripheral ligands to compete with the metal in donating electrons to the ligand-binding atom.