Ligand-stabilization of an unusual square-based pyramidal geometry of Cd(II) and Zn(II) in an heterometallic {MPt2S2} core (M = Cd, Zn)

Two heterometallic complexes, [Pt2MCl(bipy)(PPh3)4(μ3-S)2][PF6] (M = Zn, 2, Cd, 3) were synthesized from [Pt2(PPh3)4(μ-S)2] and characterized by single-crystal X-ray diffraction and electrospray ionization mass spectrometry. Two unusual square-based pyramidal (sbp) Zn(II) and Cd(II) structures are evident. VT 31P-{1H} NMR studies showed that 2 and 3 are fluxional at rt whereby rapid ligand exchange takes place by a non-dissociative mechanism. At intermediate temperatures, this motion slows down to a flipping movement of the {Pt2S2} ligand. At 183 K, all four phosphines are inequivalent in a distorted sbp model similar to that observed in the solid state. Nonlocal density functional theory calculations reveal that the formation of a trigonal bipyramidal intermediate in the fluxional process is favored over that of the tetrahedral species for both 2 and 3. The M–Cl (M = Zn, Cd) bonds are notably strong.