Coordination Chemistry Reviews 248 (2004) 683–724 Review Electronic communication in heterobinuclear organometallic complexes through unsaturated hydrocarbon bridges Alberto Ceccon ∗ , Saverio Santi, Laura Orian, Annalisa Bisello Dipartimento di Chimica Fisica ‘Arturo Miolati’, Università degli Studi di Padova, Via Loredan 2, 35131 Padova, Italy Received 16 October 2003 Available online 7 May 2004 Contents Abstract ................................................................................................................................ 683 1. Introduction ........................................................................................................................ 684 1.1. General overview ............................................................................................................. 684 1.2. Scopes and limitations ......................................................................................................... 684 1.3. Theoretical aspects ............................................................................................................ 685 2. Carbon -bonded molecular wires .................................................................................................... 687 2.1. All-carbon sp bridged molecular wires ........................................................................................... 688 2.2. Carbon based sp 2 bridged molecular wires ....................................................................................... 694 2.3. Other carbon based sp/sp 2 bridged molecular wires ............................................................................... 695 3. Fulvalene and fulvalene-like bridges .................................................................................................. 696 3.1. Fulvalene complexes .......................................................................................................... 696 3.1.1. The concept of fulvalene ................................................................................................ 696 3.1.2. Homobimetallic asymmetric fulvalene complexes ......................................................................... 697 3.1.3. Heterobimetallic fulvalene complexes .................................................................................... 698 3.2. Fulvalene-like bridges ......................................................................................................... 708 3.2.1. The phenyl-cyclopentadienyl ligand ..................................................................................... 708 3.2.2. Other fulvalene-like bridges ............................................................................................. 709 4. Fused delocalized polycyclic bridges .................................................................................................. 712 4.1. Indenyl, indacene and pentalene derivatives ...................................................................................... 712 4.2. Polycyclic derivatives .......................................................................................................... 718 5. Conclusions and perspectives ......................................................................................................... 719 Acknowledgements ...................................................................................................................... 719 References ............................................................................................................................. 719 Abstract Much of the interest about the construction of model compounds suitable for investigating the phenomena associated with the interaction of two or more metal centers, the so-called cooperative effect, has been concentrated on homobimetallic complexes and less attention has been paid to the class of heterobimetallic derivatives. This review will feature experimental detection and the effects of the electronic communication in heterobinuclear organometallic complexes through unsaturated hydrocarbon bridges. The described systems are classified according to the nature of the bridging ligand, in three main groups: (i) carbon -bonded molecular wires; (ii) fulvalene and fulvalene-like bridges; (iii) fused delocalized polycyclic bridges. In this contribution, we discuss the flexibility of heterobimetallic complexes, and more in general of asymmetric bimetallic species, in terms of tailoring the cooperative effects, i.e. of controlling and tuning the reactivity of one metal center by acting on the ad- jacent one. Satisfactory quantitative estimate of the degree of metal to metal communication through the bridging ligand is obtained in illustrative examples combining efficient electrochemical and spectroscopic techniques with consolidated theories. A review with almost 350 references. © 2004 Elsevier B.V. All rights reserved. Keywords: Heterobimetallic complexes; Electron transfer; Molecular wires; Mixed-valence chemistry; Cooperative effects Abbreviations: Bpy, bipyridine; CHT, 1,3,5-cycloheptatriene; COD, 1,5-cyclooctadiene; COT, 1,3,5,7-cyclooctatetraene; Cp, cyclopentadienyl; Cp ∗ , pentamethylcyclopentadienyl; CV, cyclic voltammetry; DMAD, dimethylacetylenedicarboxylate; dppe, 1,2-bis-(diphenylphosphino)ethane; dppf, 1,1 ′ -bis-(diphenylphosphino)ferrocene; dppm, bis(diphenylphosphino)methane; EHMO, extended Huckel molecular orbitals; ET, electron transfer; Fv, ful- valene; Ic, indacene; Ind, indenyl anion; Ind ∗ , heptamethylindenyl anion; IR/NIR, infrared/near-infrared; IVCT, inter valence charge transfer; MP, methyl- propiolate; NBD, norbornadiene; NLO, non-linear optics; TMEDA, N,N,N,N-tetramethylethylenediamine; Tp ′ , hydridotris(3,5-dimethylpyrazolyl)borate ∗ Corresponding author. Present address: Dip. Scienze Chimiche, via Marzolo 1, 35131 Padova, Italia. Fax: +39-049-8275135. E-mail address: aceccon@chfi.unipd.it (A. Ceccon). 0010-8545/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2004.02.007