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How many atoms can Th coordinate?

Recent research from the research group of Professor Laura Gagliardi.

The concept of coordination number is extremely useful and widely employed to describe the local chemical environments of atoms in matter. Originally defined by Alfred Werner in 1893, the coordination number is closely tied to many other important properties such as atomic radius, molecular and electronic structure, and chemical reactivity.

Very high Werner coordination numbers are seen for metal complexes of the borohydride anion BH4. For example, Zr(BH4)4, has a coordination numbers of twelve, while Th(BH4)4 has coordination numbers of 14. No complex of any kind, however, has been definitively shown to adopt a Werner coordination number of 15.

A joint experimental and computational work, respectively performed in the groups of Gregory S. Girolami, University of Illinois and Laura Gagliardi, University of Minnesota, has lead to the synthesis and characterization of Th(H3BNMe2BH3)4, the first 15-coordinate complex of such type. This work will appear soon in Angew. Chemie Int. Ed. as a Communication.

Both the X-ray and neutron diffraction results show that thorium forms bonds with fifteen hydrogen atoms; accordingly, this is the first crystallographically characterized complex with a Werner coordination number of 15. DFT calculations on the single Th(BH3NMe2BH3)4 unit or a finite cluster of units do not reproduce the crystal structure. However, imposing periodic boundary conditions gives a geometry for Th(BH3NMe2BH3)4 that is in a very good agreement with that determined from experiment.

The present results set a new record – fifteen – for the highest Werner coordination number seen to date in any form of matter. This finding suggests that the discovery of metal complexes with Werner coordination numbers equal to 16 should be possible with the right combination of metal and ligands.


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