Although less earth abundant than 3d transition metals, 4d transition metals often form catalysts with superior catalytic performance (e.g., Ru for water oxidation or hydrogenation ) compared to 3d transition metal catalysts. A compounding consideration is that according to hardsoft acid-base (HSAB) theory , 4d metals should prefer to be coordinated by softer 3p ligands whereas 3d metals prefer 2p ligands. Thus, we need to investigate both metal and ligand period effects simultaneously to understand changes in catalytic activity.