Different Ways To Distort a Tetracapped Tetrahedron on Route to Forming an E₄M₄ Cubane:  The Case of [E₄(Pd(PPh₂Me)₂)₄][Ph₂EX₂]₂ (E = Sb, X = Cl; E = Bi, X = Br)

Tetrakis(diphenylmethylphosphine)palladium reacts with diphenylantimony chloride or diphenylbismuth bromide to give [E4(PdL2)4][Ph2EX2]2 (L = PPh2Me, 1: E = Sb, X = Cl; 2: E = Bi, X = Br) which have been characterized spectroscopically and by single-crystal X-ray diffraction for [Sb4(PdL2)4][Ph2SbCl2]2·0.5THF and [Bi4(PdL2)4][Ph2BiBr2]2. These compounds are electron-rich based on electron counting formalisms. The additional cluster electrons can be rationalized by the ability of group 15 elements to show hypervalency, particularly those elements such as Sb and Bi which show more metal character. The electronic structure of the compounds and of related species has been examined by EHT and DFT calculations. Relationships to other cubane-derived structures are derived, and the stability of structurally related M4E4 hypothetical clusters is discussed. Compounds 1 and 2 decompose thermally to give Bi2Pd and SbPd.