NEW NITRILE NIOBOCENE COMPLEXES - ELECTROCHEMICAL STUDIES ON THEIR ETA(1) ETA(2) NITRILE ISOMERIZATION PROCESSES/

The one-electron reduction of Nb(eta(5)-C(5)H(3)RR')(2)X(2) (1:R=SiMe( 3), R'=H, X=Cl; 1'; R=SiMe(3), R'=H, X=Br; 1 '': R=R'=SiMe(3), X=Cl) i n the presence of nitrile compounds R '' CN (a:R ''=CH3; b: R ''=Ph) i nitially yields the corresponding eta(1)-nitrile-containing niobium(II I) complexes;Nb(eta(5)-C(5)H(3)RR')(2) X(eta(1)-R '' CN) (3a, 3'a, 3 ' ' a, and 3 '' b) identified by electrochemical methods. These species evolve at room temperature to the more thermodynamically stable eta(2) -nitrile-containing complexes, Nb(eta(5)-C(5)H(3)RR')(2)X(eta(2)-R '' CN) (5a, 5'a, 5 '' a and 5 '' b). complexes 5a and Nb(eta(5)-C(5)H(4)S iMe(3))(2)Cl(eta(-)Ph CN) 5b were alternatively prepared and character ized from the chemical reaction of Nb(eta(5)-C(5)H(4)SiMe(3))(2)Cl 2 w ith a and b, respectively. Kinetic studies from electrochemical data h ave allowed us to establish the rate constant for the 3a-->5a isomeriz ation process at 7.31 x 10(-5) and 5.75 x 10(-4) s(-1) at 0 degrees C and 28 degrees C, respectively (activation energy: Delta E#=55.83: kJm ol(-1)). Finally, a reverse isomerization process (eta(2)-->eta(1) nit rile) has been observed for the paramagnetic cationic niobium (IV) com plexes, [Nb(eta(5)-C(5)H(3)RR')(2)X(eta(2)-R '' CN)](+) 6 and [Nb(eta( 5)-C(5)H(3)RR')(2)X(eta(1)-R '' CN)](+) 4, generated from electrochemi cal oxidation of the corresponding niobium (III) derivatives 5 and 3.