International audienceDFT calculations with full geometry optimization have been carried out on a series of real and hypothetical compounds of the type LnMAz, MAz2, LnM2Az, and M2Az2 (Az = azulene). The analysis of their electronic and molecular structures in relation to their electron counts allows a comprehensive rationalization of the bonding within this very large family of compounds. A very rich coordination chemistry of azulene is apparent, even much richer than one could determine from the avalaible experimental data. The reason for this diversity comes in part from the marked dissymmetry of azulene, which is made up of two fused rings of very different sizes. It comes also in large part from the very large electronic and structural flexibility of azulene (in contrast to its isomer naphthalene), which is able to adapt to the electronic demand of the metal(s). Any of the fused C5 and C7 rings of azulene can be coordinated in various hapticities and symmetries, depending on the nature of the MLn moiety (or moieties) they are bonded to. This flexibility favors the possibility of existence of several isomers (sometimes enantiomers) of similar energy and of their interconversion in solution, in particular through haptotropic shifts. The azulene asymmetry causes dinuclear complexes to exhibit very different coordination environments (sometimes different oxidation states). In some of them, M−M bonding is preferred over M−azulene bonding. Most of the investigated complexes are expected to exhibit a rich fluxional behavior

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