Pd<sub>3</sub> Sandwich Complexes with π‑Cyclic\nLigands: Theoretical Insight into Stability, Face-Capping Coordination\nBond, Trans-Influence, and Backside-Ligand Effect

Triangle Pd<sub>3</sub> sandwich complexes with two face-capping\nπ-cyclic ligands, [Pd<sub>3</sub>(Cm)(Cn)L<sub>3</sub>]<sup>2+</sup> [L = acetonitrile; Cm, Cn = benzene, cycloheptatriene (CHT),\ncyclooctatetraene (COT), and [2.2]<i>para</i>cyclophane\n(PCP)], are theoretically investigated. The stability increases in\nthe order [Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>L<sub>3</sub>]<sup>2+</sup> ∼ [Pd<sub>3</sub>(C<sub>6</sub>H<sub>4</sub>Me<sub>2</sub><sup>P</sup>)<sub>2</sub>L<sub>3</sub>]<sup>2+</sup> (C<sub>6</sub>H<sub>4</sub>Me<sub>2</sub><sup>P</sup> = <i>p</i>-xylene) < [Pd<sub>3</sub>(COT)<sub>2</sub>L<sub>3</sub>]<sup>2+</sup> < <b>[Pd<sub>3</sub>(CHT)<sub>2</sub>L<sub>3</sub>]<sup>2+</sup></b> < <b>[Pd<sub>3</sub>(PCP)<sub>2</sub>L<sub>3</sub>]<sup>2+</sup></b> and [Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>L<sub>3</sub>]<sup>2+</sup> ∼ [Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)(C<sub>6</sub>H<sub>4</sub>Me<sub>2</sub><sup>P</sup>)L<sub>3</sub>]<sup>2+</sup> < [Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)(PCP)L<sub>3</sub>]<sup>2+</sup> <\n[Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)(CHT)L<sub>3</sub>]<sup>2+</sup> < <b>[Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)(COT)L<sub>3</sub>]<sup>2+</sup></b> < [Pd<sub>3</sub>(CHT)(PCP)L<sub>3</sub>]<sup>2+</sup> < <b>[Pd<sub>3</sub>(COT)(PCP)L<sub>3</sub>]<sup>2+</sup></b>, where “bold” represents an experimentally\nisolated complex. Based on these results, we succeeded in synthesizing\na new complex [Pd<sub>3</sub>(CHT)(PCP)L<sub>3</sub>]<sup>2+</sup>. Geometrical features suggest that benzene and PCP interact with\none Pd atom using one CC double bond and with two other Pd\natoms using the other CC double bond in the bridging manner.\nCHT interacts with three Pd atoms using three CC double bonds.\nCOT interacts with three Pd atoms using three CC double bonds\nin [Pd<sub>3</sub>(COT)<sub>2</sub>L<sub>3</sub>]<sup>2+</sup> and\nusing π-allyl type MOs with additional participation of two\nCC π bonds in [Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)(COT)L<sub>3</sub>]<sup>2+</sup>. CHT and PCP weaken the benzene\ncoordination bond in [Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)(CHT)L<sub>3</sub>]<sup>2+</sup> and [Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)(PCP)L<sub>3</sub>]<sup>2+</sup> like trans-influence. Unprecedentedly,\nCOT strengthens the benzene coordination bond in [Pd<sub>3</sub>(C<sub>6</sub>H<sub>6</sub>)(COT)L<sub>3</sub>]<sup>2+</sup>. This “backside-ligand\neffect” arises from the characteristic coordination bond of\nthe COT, which is elucidated based on the orbital interaction between\nthe COT and Pd<sub>3</sub> triangle.