The MO diagram is given above. The orbitals in the blue box
are the frontier orbitals. They are neither strongly bonding, nor strongly antibonding, and in fact very strongly resemble the
frontier orbitals of an octahedral transition-metal complex. The e2’ and a1’ together are closely allied to the Oh t2g set, while the
e1” directly corresponds to the eg set. This accounts for the variable electron-count and allows for a variety of electron
configurations, including high- and low-spin variants of MnCp2
This also rationalizes the chemical behaviour of the various metallocenes, those with access electrons are easily oxidized, the
others are easily reduced. Only the 18-electron examples are air-stable solids.
Electronic configuration and M-C bond lengths in M(η5-C5H5)2 complexes
Complex Valence electrons Electron
configuration
R(M-C)/pm
V(C5H5)2 15 (e2’)2 (a1’)1 228
Cr(C5H5)2 16 (e2’)3 (a1’)1 217
Mn(C5H4CH3)2
* 17 (e2’)3 (a1’)2 211
Fe(C5H5)2 18 (e2’)4 (a1’)2 206
Co(C5H5)2 19 (e2’)4 (e1”)1 (a1’)2 212
Ni(C5H5)2 20 (e2’)2 (e1”)2 (a1’)2 220
* Data are quoted for this complex because Mn(C5H5)2 has a high-spin
configuration: (a1’)2 (e2’)2 (e1”)2 and, hence an anomalously long M-C bond
length (238 pm) .