Abstract
(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)
2NO]
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 dissociatcs CO in the rate limiting step with the following activation parameters: Δ
H
≠=60±6 kJ; δ
S≠=+ 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)
2NO
+ (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)
2NO
+ is reduced by orte 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.