The potent electron‐donating group [(CpFeCO)2(µ‐CO)(µ‐C=CH−)] was combined with different tropylium‐based acceptors yielding the push‐pull complexes [(CpFeCO)2(µ‐CO)(µ‐C=CH−CH=A)][BF4] where =A represents [=CH−(η7‐C7H6)Cr(CO)3]+ (5), [={C10H7}]+ (azulenylium) (7) [={C10H4Me2iPr}]+ (guaiazulenylium) (8). Crystal structures of the precursor complexes [(CpFeCO)2(µ‐CO)(µ‐C=CH−CH=CH−C7H7)] (3), [(CpFeCO)2(µ‐CO){µ‐C=CH−CH=CH−(η6‐C7H7)Cr(CO)3}] (4) and the push‐pull derivative 5 are presented. Whereas complexes 3 and 4 demonstrate localised carbon−carbon double and single bonds in the π‐bridge between the diiron moiety and the seven‐membered ring a small but distinct π‐bond delocalisation can be observed in the π‐linker of the cationic derivative 5. From the NMR spectroscopic results a correlation between the 13C NMR shift of the bridging carbon atom µ‐C of the diiron unit and the coupling constant J(1H‐1H) of the β‐ and γ‐protons of the π‐bridge was found. This correlation indicates an increased π‐bond delocalisation throughout the conjugated bridge with respect to the electron‐accepting capability in the order tricarbonyl(η7‐cycloheptatrienylium)chromium < guaiazulenylium < azulenylium implying a predominantly charge‐delocalised form in the case of the azulenylium complex 7. Considerably large first hyperpolarisabilities β were determined for 5 and 8 by means of hyper‐Rayleigh scattering. Based on the two‐level model β0 values were calculated and related to the bond‐length alternation in the π‐linker. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

The ionic cleavage of Ti-Co and Zr-Co bonds occurs upon reaction with Bu t NC and the products [MeSi{SiMe 2 N(p-Tol)} 3 Ti(CNBu t ) 2 ][Co(CO) 4 ] 3 and[MeSi{SiMe 2 N(p-Tol)} 3 Zr(CNBu t ) 3 ][Co(CO) 4 ] 4 were characterized by single crystal X-ray diffraction; the importance of the nucleophilicity of the late transition metal fragment in reactions of the unsupported early-late heterobimetallics is demonstrated.

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