_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Five electron ligands
Dihalogenomethanes give cyclohexadienyl salts in which the CH 2 group has
inserted into one ofthe cyclopentadienyl rings. A similar reaction with arylboron
dihalides affords complexes which contain the borabenzene ligand C 5 H 5 B-R.
This ligand provides a six-membered delocalized system but only five ligand n­
electrons.
Electrochemical reduction of Cp 2 Co + proceeds in two 1-electron steps at
-0.945 V and -1.88 V relative to the standard calomel electrode. The final
product Cp 2 Co- is so strongly nucleophilic that it attacks even carbon dioxide
(cf. a Grignard reagent). This reduction can also be effected by potassium in
tetrahydrofuran. The Cp- anion is quite a good leaving group, so in the presence
of suitable ligands such as alkenes or CO, K: [CoCp_;] affords KCp and
monocyclopentadienyl complexes CpCoL 2 • This general strategy has been used to
prepare monocyclopentadienyl derivatives from other metallocenes (see Fig. 9. 6 ).
Nickelocene, a 20-electron compound, has two electrons in the antibonding
e~g orbitals. The monocation Cp 2 Ni + can be generated chemically with halogens
or with dilute nitric acid. Electrochemical oxidation occurs in two steps to
Cp 2 NF + through Cp 2 Ni +. Reduction to Cp 2 Ni- has also been achieved.
SUBSTITUTION
CpCo(PR3)2
CpCo(COl2
REDUCTION
K+[ Cp2cof
2C2 H4j-KCp
CpCo (C 2H4 )2
-e-
+e-
+e-
+ -
Cp 2 Co X
+
PR 3 Co or
~-R ~8-R
Co + Co
®R-~
BORABENZENE
COMPLEXES
Fig. 9.6 Some reactions of cobaltocene.
288
Cp2 Co X
-e-
OXIDATION
+- JH
Cp2Co X + ~
+- rr1, ~~
+ V
co+
Cp