Stabilizing Ca-H

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Science  12 May 2006:
Vol. 312, Issue 5775, pp. 817
DOI: 10.1126/science.312.5775.817b

The s-block metals, whose valence electrons lie exclusively in s orbitals, are widely known for their ionic chemistry. Through careful ligand choice, metals such as sodium, magnesium, and calcium can also be coaxed into discrete coordination complexes. However, s-block molecular hydride complexes, which are of particular interest in light of the strong role of p- and d-block metal hydrides in organic reduction chemistry, have proven challenging to access, because they tend to decompose into insoluble aggregates.

Harder and Brettar have prepared a dimeric calcium hydride complex that is freely soluble in benzene and stable at 80°C. The solid-state structure, in which two hydride ligands bridge the two Ca centers, was characterized by x-ray crystallography. Key to the synthesis was the choice of a tightly coordinating β-diketiminate ancillary ligand on each Ca center. Surprisingly, the bulky tris(tert-butylpyrazolyl)borate (TptBu) ligand failed to prevent disproportionation into (TptBu)2Ca and the insoluble CaH2 oligomer, despite stabilizing a hydride complex of calcium's lighter congener beryllium. — JSY

Angew. Chem. Int. Ed. 45, 10.1002/anie.200601013 (2006).

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