The interaction of pyridine with 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), have been studied in 2,2,2-trifluoroethanol medium. The kinetics of the reactions were investigated over the 293-318 K temperature range. The reaction profile exhibited two distinct processes, proposed to be an initial fast substitution followed by a slow reduction, for each of the reactions. The pseudo first-order rate constants for both processes increased with increasing concentration of pyridine with the reduction processes exhibiting saturation kinetics at high pyridine concentrations. Steric hindrance plays a significant role in the rates of the reactions, as the rates decrease in the order k(A) > k(B) > k(C) > k(D). The activation enthalpies, DH à , increase from A to D while the activation entropies, DS à , are relatively similar for the four reactions, indicating similar transition states and hence similar mechanisms. Complex B was first synthesized and characterized in this study.

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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.