Pd₃ Sandwich Complexes with π‑Cyclic Ligands: Theoretical Insight into Stability, Face-Capping Coordination Bond, Trans-Influence, and Backside-Ligand Effect

Triangle Pd₃ sandwich complexes with two face-capping π-cyclic ligands, [Pd₃(Cm)(Cn)L₃]²⁺ [L = acetonitrile; Cm, Cn = benzene, cycloheptatriene (CHT), cyclooctatetraene (COT), and [2.2]paracyclophane (PCP)], are theoretically investigated. The stability increases in the order [Pd₃(C₆H₆)₂L₃]²⁺ ∼ [Pd₃(C₆H₄Me₂^P)₂L₃]²⁺ (C₆H₄Me₂^P = p-xylene) < [Pd₃(COT)₂L₃]²⁺ < [Pd₃(CHT)₂L₃]²⁺ < [Pd₃(PCP)₂L₃]²⁺ and [Pd₃(C₆H₆)₂L₃]²⁺ ∼ [Pd₃(C₆H₆)(C₆H₄Me₂^P)L₃]²⁺ < [Pd₃(C₆H₆)(PCP)L₃]²⁺ < [Pd₃(C₆H₆)(CHT)L₃]²⁺ < [Pd₃(C₆H₆)(COT)L₃]²⁺ < [Pd₃(CHT)(PCP)L₃]²⁺ < [Pd₃(COT)(PCP)L₃]²⁺, where “bold” represents an experimentally isolated complex. Based on these results, we succeeded in synthesizing a new complex [Pd₃(CHT)(PCP)L₃]²⁺. Geometrical features suggest that benzene and PCP interact with one Pd atom using one CC double bond and with two other Pd atoms using the other CC double bond in the bridging manner. CHT interacts with three Pd atoms using three CC double bonds. COT interacts with three Pd atoms using three CC double bonds in [Pd₃(COT)₂L₃]²⁺ and using π-allyl type MOs with additional participation of two CC π bonds in [Pd₃(C₆H₆)(COT)L₃]²⁺. CHT and PCP weaken the benzene coordination bond in [Pd₃(C₆H₆)(CHT)L₃]²⁺ and [Pd₃(C₆H₆)(PCP)L₃]²⁺ like trans-influence. Unprecedentedly, COT strengthens the benzene coordination bond in [Pd₃(C₆H₆)(COT)L₃]²⁺. This “backside-ligand effect” arises from the characteristic coordination bond of the COT, which is elucidated based on the orbital interaction between the COT and Pd₃ triangle.