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Molecular alloys: Experimental and theoretical investigations on the substitution of zinc by cadmium and mercury in the homologous series [Mo(M′R)<inf>12</inf>] and [M(M′R)<inf>8</inf>] (M=Pd, Pt; M′=Zn, Cd, Hg)

Abstract:

The synthesis and structural characterization of novel, metal-rich, highly coordinated compounds [Mo(M′R)12] and [M(M′R) 8] (M: Pd, Pt, Mo; M′: Zn, Cd; R: Me=CH3, Cp*=pentamethylcyclopentadienyl) are reported. Additionally, a description of the bonding situation of the new compounds by means of quantum-chemical calculations is presented including the Hg analogues. Reaction of [Pt(GaCp*)4] with CdMe2 results in the formation of the unprecedented all-Cd coordinated [Pt(CdMe)4(CdCp*) 4] (1). Similarly, the treatment of the all-Zn coordinated [Pd(ZnMe)4(ZnCp*)4] with CdMe2 affords the novel Zn/Cd mixed compound [Pd(CdMe)4(ZnCp*)4] (2). The related Zn/Cd mixed compound [Mo(ZnCp*)3(CdMe) 9] (3) is prepared by reaction of [Mo(ZnCp*) 4(GaMe)4] with an excess amount of CdMe2. All compounds were analyzed by 1H and 13Ca NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The bonding situation of these highly coordinated, metal-rich molecules 1-3 were studied by quantum-chemical calculations using density functional theory (DFT) at the BP86/TZ2P+ level, atoms-in-molecules (AIM) analysis, and energy-decomposition analysis (EDA), as well as the its natural orbitals for chemical valence variation (EDA-NOCV) and including the hypothetically all-Hg-coordinated analogues. The results point out that the radial interactions M-M′ in the icosahedral compounds that have twelve ligands are best described as classical electron-pair-sharing covalent bonds, whereas the dodecahedral species, which have eight ligands, exhibit metal-ligand donor-acceptor bonds. The attractive interactions between the metal-ligand fragments M′R by means of M′-M′ bonds are weaker but not insignificant. All complexes fulfill the 18-electron rule. The analysis clarifies the electronic structures as being distinctly different from typical endohedral clusters M at the (M″R) n that exhibit strong peripheral M″-M″ interactions: The M′-M′ bonds are not strong enough to yield stable (M′R) n cages. Coordination or cluster compound? Reactions of transition-metal precursors that contain GaCp* and/or ZnIR (R=CH3, Cp*) ligands with CdMe2 lead to novel molecules that exhibit monovalent CdR ligands. The picture shows the metal cores of the new compounds. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.