The chiral monodentate phosphine PhP[(C5Me4)(2)] is readfly obtained by oxidation of the lithium complex Li-2[PhP(C5Me4)(2)] with I-2, which couples the two cyclopentadienyl groups to form a five-membered heterocyclic ring. The steric and electronic properties of PhP[(C5Me4)(2)] have been evaluated by X-ray diffraction and IR spectroscopic studies on a variety of derivatives, including Ph[(C5Me4)(2)]PE (E = S, Se), Cp*MCl4{P[(C5Me4)(2)]-Ph} (M = Mo, Ta), Ir{P[(C5Me4)(2)]Ph}(2)(CO)Cl, and CpFe(CO){PhP[(C5Me4)(2)]}Me. For comparison purposes, derivatives of the related phospholane ligand PhP[Me2C4H6] have also been investigated, including Ph[Me2C4H6]-PS, Ir{Ph[Me2C4H6]}(2)(CO)Cl, Ir{Ph[Me2C4H6]}(2)(CO)Me, Ir{PPh[Me2C4H6]}(COD)(Cl), and Pd{P[Me2C4H6]-Ph}[eta (2)-C6H4C(H)(Me)NMe2]Cl. The steric and electronic properties of PhP[(C5Me4)(2)] are determined to be intermediate between those of PPh2Me and PPh3. Thus, the crystallographic cone angles increase in the sequence PPh2Me (134.5 degrees) < PhP[(C5Me4)(2)] (140.2 degrees) < PPh3 (148.2 degrees), while the electron donating abilities decrease in the sequence PPh2Me > PhP[(C5Me4)(2)] > PPh3. Finally, PhP[(C5Me4)(2)] has a smaller cone angle and is less electron donating than the structurally similar phosphine, PhP[Me2C4H6].