Tetrazines react with OCP-1 through a reverse electron demand Diels-Alder process to produce 3,6-disubstituted-1,2,4-diazaphosphinin-5-olates. DFT calculations reveal that both Diels-Alder and subsequent aromatization barriers are low for both EWG and ED tetrazine substituents. The structure of the solid sodium salt shows the interaction of Na+ with aryloxide and also both nitrogens of a neighboring anion, leading to coordination polymer character. 1,2,4-Diazaphosphinin-5-olates react as nucleophiles towards MeI and R3SiCl, respectively, and were installed on the (Ph3P)2Ru(CO)H fragment to investigate their properties as ligands.