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(C6H4Me2P)2L3]2+ (C6H4Me2P = p-xylene) < [Pd3(COT)2L3]2+ < [Pd3(CHT)2L3]2+ < [Pd3(PCP)2L3]2+ and [Pd3(C6H6)2L3]2+ ∼ [Pd3(C6H6)(C6H4Me2P)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.