Web Date: November 5, 2008
Molybdenum Takes On A Dozen Zincs
Pushing the known boundaries of bonding in transition-metal compounds, a European research team has synthesized a highly coordinated complex containing a MoZn12 core (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200802811). The unusual Mo[Zn12(CH3)9(Cp*)3] molecule, where Cp* is the pentamethylcyclopentadienyl ligand, appears to have 12 metal-ligand bonds and provides a bridge between traditional transition-metal coordination compounds and main-group cluster compounds.
The team led by Thomas Cadenbach and Roland A. Fischer of Ruhr University, in Bochum, Germany, and Gernot Frenking of Philipps University, in Marburg, Germany, made the compound in 80% reproducible yield by reacting Mo(GaCp*)6 and Zn(CH3)2.
The complex obeys the 18-electron rule, a rule of thumb in transition-metal chemistry stating that complexes are most stable when 18 valence electrons are involved in the bonding. This rule typically limits the highest coordination number of ligands around a metal center to nine for standard two-electron bonds.
But on the basis of spectroscopic, X-ray crystal structure, and computational studies, the researchers propose that molybdenum's six valence electrons together with six of the zinc valence electrons are engaged in six Zn–Mo–Zn "three-center, two-electron bonds," effectively forming 12 one-electron Mo–Zn bonds. The remaining six zinc valence electrons are delocalized over the zinc atoms surrounding molybdenum, forming weak Zn–Zn bonds.
The complex shares similarities with so-called Zintl compounds, in which metal atoms are trapped within main-group cluster anions. These "endohedral cages" usually involve a transition-metal atom surrounded by a framework of more than nine atoms of an element in group 13 to 16, such as lead.
Isolated molecular compounds with coordination numbers as high as 12 have been observed only for lanthanide and actinide elements, says Peter Schwerdtfeger, a theoretical chemist at Massey University, in Auckland, New Zealand, who has investigated high-coordination metal complexes. One known complex, U(BH4)4, has a coordination number of 14, he notes. But the MoZn12 complex involving such a highly coordinated transition metal is unprecedented, making it "quite a jewel," Schwerdtfeger says.
Bryan Eichhorn, a chemistry professor at the University of Maryland whose research includes Zintl compounds, adds that there are few examples of molecules like the MoZn12 cluster. The synthesis of the new compound has "far-reaching implications" for its magnetic properties and its potential as a superconducting material, he says.
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