Charged molecular alloys: Synthesis and characterization of the binary anions Pd₇As₁₆^4- and Pd₂As₁₄^4-

Pd(PCy₃)₂ (Cy = cyclohexyl) reacts with As₇^3- in en/tol solvent mixtures to give Pd₂As₁₄^4- (2) and Pd₇As₁₆^4- (4) as the [K(2,2,2-crypt)]⁺ salts. The anions were characterized by EDX, ESI-MS, and single-crystal X-ray diffraction. Anion 2 formally contains two norbornadiene-like As₇^5- groups bound to square-planar Pd(III) centers linked by a Pd-Pd bond (d_Pd-Pd = 2.7144(6) Å). Anion 4 has a highly distorted capped trigonal prismatic Pd₇ core stabilized by 2 As₅-¹, 2 As₂-², and 2 As-³ anions. The 6 Pd(I) ions are in distorted 5-coordinate environments whereas the lone Pd(II) ion is square planar. Complexes 2 and 4 are rare examples of organic-free, homoleptic transition metal anions containing group 15 elements, and they represent an emerging class of charged "molecular alloys".