Substitution and Redox Properties of Some Organoisocyanide Cobalt(II) Complexes

Abstract

The reactions of four tetrakis(arylisocyanide)cobalt(II) complexes, [Co(CNR) 4 (ClO 4 ) 2 ] {R = 2,6-Me 2 C 6 H 3 (A), 2,4,6-Me 3 C 6 H 2 (B), 2,6-Et 2 C 6 H 3 (C), and 2,6-iPr 2 C 6 H 3 (D)}, with two pyridines, 4-CNpy and 4-Mepy, have been kinetically studied in trifluoroethanol medium. Each of the reactions, which was monitored over a temperature range of 293 to 318 K, exhibited two distinct processes proposed to be an initial fast substitution process followed by a slow reduction process. For each pyridine, steric hindrance was observed to play a significant role in the rates of the reactions, which decrease with increasing size of the arylisocyanide ligand in the order k(A)>k ( B)>k (C)>k ( D). Addition of each of three triarylphosphines, PR 3 (R = Ph, C 6 H 4 Me-p, C 6 H 4 OMe-p), to solutions of pentakis (t-octylisocyanide)cobalt(II), [Co(CNC 8 H 17 -t) 5 ](ClO 4 ) 2 , resulted in a shift in the λ max of the electronic spectrum accompanied by a change in color of the solutions. The shift is attributed to ligand substitution. The reactions of the cobalt(II) complex [Co(CNC 8 H 17 -t) 5 ] 2+ with the triarylphosphines are proposed to proceed via a combination of substitution, reduction, and disproportionation mechanisms with final formation of disubstituted Co (I) complexes. The order of reactivity of the complex with the triarylphosphines was found to be P(C 6 H 4 OMe-p) 3 >P ( C 6 H 4 Me-p) 3 >P P h 3 . This order is explained in terms of the electron donating/π-acceptor properties of the phosphines.