Kinetics and mechanisms of the ligand substitution of (η5-C9H7)M(CO)4, where M=Nb or Ta, and a reinvestigation of the kinetics of (η5-C5H5)M(CO)4. Molecular structure of (η5-C5H5)Nb(Co)3[P(C6H5)3]

Kinetic studies were performed for CO substitution reactions of (η5-C5H5)M(CO)4 and (η5-C9H7)M(CO)4 (where M=Nb, Ta) with phosphines and phosphites. Niobium derivatives were found to undergo ligand exchange at higher rates than tantalum, and indenyl derivatives exchanged faster than cyclopentadienyl derivatives. The cyclopentadienyl compounds were found to react by a dissociative or dissociative interchange mechanism as opposed to an associative mechanism as suggested by an earlier kinetic study. The indenyl compounds were found to predominantly react by an associative mechanism. Rate and thermodynamic data indicate that the indenyl effect is operative in these Group V compounds. The molecular structure of (η5-C5H5)Nb(CO)3[P(C6H5)3]·0.25CH2Cl2 was determined: P1̄, a=10.817(2), b=12.403(2), c=18.962(3) Å, α=100.901(13), β=104.695(10), γ=103.52(2)°, V=2307.6(7) Å3, Z=4, RF=3.36.