Molecular alloys: experimental and theoretical investigations on the substitution of zinc by cadmium and mercury in the homologous series [Mo(M'R)12] and [M(M'R)8] (M=Pd, Pt; M'=Zn, Cd, Hg).

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=CH(3), 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 CdMe(2) 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 CdMe(2) 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 CdMe(2). All compounds were analyzed by (1)H and (13)C 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@(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.