Isolation and Structural Characterization of a Mackay 55-Metal-Atom Two-Shell Icosahedron of Pseudo‑<i>I</i>_<i>h</i> Symmetry, Pd₅₅L₁₂(μ₃‑CO)₂₀ (L = PR₃, R = Isopropyl): Comparative Analysis with Interior Two-Shell Icosahedral Geometries in Capped Three-Shell Pd₁₄₅, Pt-Centered Four-Shell Pd–Pt M₁₆₅, and Four-Shell Au₁₃₃ Nanoclusters

We present the first successful isolation and crystallographic characterization of a Mackay 55-metal-atom two-shell icosa­hedron, Pd55L12­(μ3-CO)20 (L = PPri3) (1). Its two-shell icosa­hedron of pseudo-Ih symmetry (without isopropyl substituents) enables a structural/bonding comparison with interior 55-metal-atom two-shell icosa­hedral geometries observed within the multi-shell capped 145-metal-atom three-shell Pd145­(CO)72­(PEt3)30 and 165-metal-atom four-shell Pt-centered (μ12-Pt)­Pd164‑xPtx­(CO)72­(PPh3)20 (x ≈ 7) nanoclusters, and within the recently reported four-shell Au133(SC6H4-p-But)52 nanocluster. DFT calculations carried out on a Pd55­(CO)20(PH3)12 model analogue, with triisopropyl phosphine substituents replaced by H atoms, revealed a positive +0.84 e charge for the entire Pd55 core, with a highly positive second-shell Pd42 surface of +1.93 e.