Many organoisocyanide cobalt(I,II, & III) complexes that have been reported in literature exist as five-coordinate complexes. While the thermodynamics and kinetics of complex formation and ligand substitution of octahedral transition metal complexes have been widely studied, similar studies on five-coordinate square pyramidal or trigonal bipyramidal transition metal complexes have received significantly less attention. This study looks at the reactions of some five-coordinate isocyanide cobalt(II) complexes with the aim of providing information on the relative rates and mechanisms of the reactions.
The progress of the reactions of [Co(CNC8H17-t)5](ClO4)2 (A) and [Co2(CNCHMe2)10](ClO4)4.5H2O {reacting as [Co(CNCHMe2)5]2+ (B) in solution} with PPh3 and P(C6H4Me-p)3 was monitored by both infrared and UV-Visible spectroscopic techniques. Both Co(II) complexes react with the tri-aryl phosphines leading to complex kinetics. In all reactions, a fast substitution of A and B by the phosphines is first observed and this is followed by slower reduction of the substituted Co(II) to Co(I). After the substitution, a 4:1(isocyanide:phosphine) Co(I) complex is apparently formed initially and then undergoes further substitution with excess phosphine to produce a final 3:2(isocyanide:phosphine) pentakis Co(I) complex. The isopropylisocyanide-cobalt(II) complex B reacts with P(C6H4Me-p)3 leading to immediate disproportionation of B to Co(III) and Co(I). The final product is however a 3:2 Co(I) complex.
Kinetic studies on the slower reduction of the complexes to Co(I) produce complex results as it is evident that the reactions involve disproportionation and re-arrangement among others. However the first fast substitution process was followed using the stopped-flow technique. Steric hindrance seems to play a significant role in these reactions as B reacts faster with the phosphines than A does. For example, at 25oC, B reacts with PPh3 with second order rate constant (k2) of (1.29±0.03) x 103 M-1 s-1 while A reacts with the same phosphine with k2 = (9.42±0.16) x 102 M-1 s-1.