Credit: © 2009 ACS

Complexes where the central transition metal is coordinated by three ligands have different properties compared with those with the more usual four or six ligands. Stabilizing these unusual compounds normally requires all the ligands to be sterically bulky to protect the under-coordinated metal centre.

Now, Patrick Holland and colleagues from the University of Rochester have created1 a tri-coordinate cobalt complex in which one of the ligands is a hydride ion — the smallest possible ligand. They reacted a previously known three-coordinate cobalt halide with KHBEt3 and formed some unusual compounds. Using one equivalent resulted in a dimeric complex bridged by two μ2-hydrides. The cobalt atoms are close enough together to contribute to the bonding.

Two equivalents gave a more unusual structure, with two nearly parallel three-coordinate cobalt units. The two hydride ions each bridge a central K2 unit, which itself interacts with the π systems of the bulkier ligands. In the absence of cobalt–cobalt bonding, these interactions stabilize the structure. Both complexes undergo reductive elimination of hydrogen in the presence of nitrogen, with the extra electrons going to form a [N=N]2− ligand that takes the place of the bridging hydrides. Beyond the intrinsic interest of these unusual complexes, this is also relevant to catalytic nitrogen reduction, in this case avoiding the use of harsh reducing agents.