LIGAND SUBSTITUTION AT 19-ELECTRON ORGANOMETALLIC CENTERS - ELECTROCATALYTIC CO SUBSTITUTION-REACTIONS OF (METHYLCYCLOPENTADIENYL)MN(CO)(2)NO+ AND (INDENYL)MN(CO)(2)NO+

(Methylcyclopentadienyl)Mn(CO)NO+(1(+)) undergoes a one-electron reduction at an electrode to give a 19-electron neutral radical that rapidly dissociates CO and dimerizes to yield [(MeCp)Mn(CO)(2)NO](2). In the presence of P-donor nucleophiles (L), the reduction of 1(+) initiates a rapid electron transfer catalyzed (ETC) CO substitution to give a quantitative yield of (MeCp)Mn(CO)(L)NO+. The substitution reaction occurs via the 19-electron intermediate 1, which dissociates CO in the rate limiting step with the following activation parameters: Delta H-not equal = 60+/-6 kJ: Delta S-not equal = +37+/-15 J K-1. The 17-electron intermediate (MeCp)Mn(CO)NO is then trapped by the nucleophile to give the electron rich (MeCp)Mn(CO)(L)NO, which spontaneously transfers an electron to 1(+) to afford the final product and regenerate 1. A variety of electrochemical techniques, including low temperature voltammetry and steady-state voltammetry with microelectrodes, was employed to quantitatively define the details of the reaction mechanism. The indenyl analogue of 1(+), (indenyl)Mn(CO)(2)NO+ (2(+)), was found to undergo ETC substitution reactions by the same dissociative mechanism and at approximately the same rate as 1(+). The conclusion is that the 'indenyl effect' does not operate in these 19-electron complexes. The rhenium complex CpRe(CO)(2)NO+ is reduced by one electron to give a relatively stable neutral radical that does not react with P-donor nucleophiles on the voltammetric time scale of 0.5 V s(-1). The conclusion is that CO dissociation from 19-electron complexes follows the reactivity order Mn>> Re.