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A hexagonal planar complex

21 October 2019 - Inorganic chemistry

A hexagonal planar transition metal complex is reported from the Crimmin Group (Marti Garçon et al. DOI). In textbook chemistry any metal with 6 ligands either adopts a Trigonal prismatic molecular geometry or a Octahedral molecular geometry geometry so this arrangement is unusual. The new compound has a central palladium atom with three hydride ligands and three magnesium ligands in a planar alternating fashion. It was synthesized in two steps. In step 1 CpPd(cinamyl) which is cyclopentadienyl allyl palladium with the allyl group replaced by a cinnamyl group was reacted with an aluminum hydride with a nacnac ligand forming (PdAl(H)2nacnac)2 (for simplicity the nacnac ligand is called nacnac). This intermediate was then reacted with MgH(nacnac), another magnesium compound with bridging hydrogen atoms. Single-crystal X-ray diffraction of the resulting hexagonal product PdH3(Mg(nacnac))3 showed that the maximum deviation from the plane was only 10 degrees and that the Mg-H distance was too large for bond formation.
Hydrogen ligands are electron donating and sigma donation is more efficient in a equatorial fashion that an axial fashion. As this is not compatible with the high oxidation state of palladium the authors correctly suspected that alternating sigma-donors (hydrogen is expected to have a negative charge) and sigma-acceptors (Mg with a positive change) could do the trick.