 1,2-bis(diphenylphosphino)ethane

The diphosphine $\mathrm{P}{\mathrm{h}}_2\mathrm{PC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{PP}{\mathrm{h}}_2$. Chelation to one metal makes a five-membered ring.