Spectrophotometric and conductometric studies on the interaction between trivalent phosphines and 2,2′-bipyridyl in solution

Complex formation of PCl 3 , C 6 H 5 PCl 2 , (C 6 H 5 ) 2 PCl, (C 6 H 5 ) 3 P, ( m -tolyl) 3 P, ( p -tolyl) 3 P, P(OCH 3 ) 3 , C 6 H 5 P(OCH 3 ) 2 , and (C 6 H 5 ) 2 POCH 3 with 2,2′-bipyridyl have been examined in 1,2-dichloroethane and/or acetonitrile. The continuous variation method or the mole ratio method in the ultraviolet absorption spectra shows that these phosphines form the 1:1 complexes with 2,2′-bipyridyl in solution, except for P(OCH 3 ) 3 , C 6 H 5 P(OCH 3 ) 2 , and (C 6 H 5 ) 2 POCH 3 . The measurements of electric conductivity and infrared spectra indicate that the complexes of PCl 3 , C 6 H 5 PCl 2 , (C 6 H 5 ) 2 PCl dissociate more or less into ionic species in acetonitrile, while those of R 3 P (R = phenyl, m- and p-tolyl) exist as simple addition compounds in 1,2-dichloroethane. The stability constants of these complexes estimated spectrophotometrically are the order of; PCl 3 > C 6 H 5 PCl 2 > (C 6 H 5 ) 2 PCl > (C 6 H 5 ) 3 P > (m-tolyl) 3 P > (p-tolyl) 3 P ⪢ (C 6 H 5 ) 2 POCH 3 ∼ C 6 H 5 P(OCH 3 ) 2 ∼ P(OCH 3 3 . This trend suggests that the phosphines act as Lewis acids toward 2,2′-bipyridyl and they interact mainly through σ electronic system.