Ruthenium complexes containing a hypervalent phosphorus ligand, [Cp(CO) 2 Ru{P-(OC 6 H 4 E) 2 }] (E ) NH (2a), NMe (2b), or O (4a)), [Cp(CO) 2 Ru{P(OC 10 H 6 O) 2 }] (4b), and [Cp-(CO) 2 Ru{P(OC 7 H 6 O) 2 }] (4c), were prepared in the reaction of [Cp(CO) 2 Ru{P(OPh) 3 }]BF 4 (1) with the corresponding aryl alcohol or arylamine and Et 3 N or n-BuLi. The comparison of spectroscopic data of [Cp(CO) 2 M{P(OC 6 H 4 E) 2 }] (M ) Ru, Fe) (E ) NH, NMe, O) reveals that the Ru-P bonding is more polarized as M δ+ -P δ-than the Fe-P bonding. During the course of preparation of 2b, a phosphite complex [Cp(CO) 2 Ru{P(OC 6 H 4 NMe)(OC 6 H 4 NMeH)}]-BF 4 (3) was isolated, which reacted with a Lewis base to give 2b. The X-ray structures of 2b, 4b, and 3 were determined. 2b and 4b have slightly distorted trigonal-bipyramidal geometries. From the variable-temperature 31 P NMR study for 4c, the activation parameters for Berry pseudorotation around the pentacoordinate phosphorus were determined to be ∆H q ) 42.1 ( 0.5 kJ mol -1 , ∆S q ) -91.1 ( 1.6 J mol -1 K -1 , and ∆G 340 q ) 73.1 ( 0.7 kJ mol -1 . This constitutes the first determined activation parameters of Berry pseudorotation for a metallaphosphorane complex.