CLUSTERS cyclopentadienyl complexes

As mentioned above, polymorphism may also arise for compounds formed by isomers whose structures are related by low-energy interconversion pathways, e.g. cluster carbonyls with different distributions of bridging and terminal ligands, or substituted metallo-arenes and metallo-cyclopentadienyl complexes [60]. In such cases, the structural isomers correspond to different energetic minima along the interconversion pathway and the cohesion of the respective crystals may stabilize the less thermodynamically stable isomers. Crystals of structural isomers may (or may not) interconvert via a phase transition. 

The cubane-like cluster [ Fe(/X3-S)(NO) 4] (68) is not formally an organometallic compound, but the structural changes observed when it is reduced, and the MO description of its bonding, are relevant to the cyclopentadienyl complexes described above. The nitrosyl complex contains 60 valence electrons, 8 less than [ Fe()U3-S)Cp 4], and is reduced in two consecutive one-electron steps to an isolable monoanion [68] , and to a dianion [68] . 

Cyclopentadienyl complexes are often prepared as dimeric species, but less frequently as higher clusters. Several techniques are used to prepare cyclopentadienyl dimers. A generally useful reaction is of a mononuclear carbonyl with dicyclopentadiene. The proper conditions for preparation of Cp2Fe2(CO)4 have been studied . ... 

E4.3 (Cyclopentadienyl)metal cluster complexes of the Group 9 transition metals... 

This is a special volume of Inorganic Syntheses that focuses on complexes that are likely to be useful as starting materials for the preparations of new transition metal coordination and organometallic compounds. There are chapters on complexes with weakly coordinated and therefore easily displaced ligands, low-valent complexes that undergo oxidative-addition reactions, substituted metal carbonyl complexes, nucleophilic metal carbonyl anions, transition metal clusters, a variety of cyclopentadienyl complexes, lanthanide and actinide complexes, and a range of other useful ligands and complexes. 

Figure 9.11, Structures of cluster cyclopentadienyl complexes (a) Co4Cp4H4 and (b) Rli3Cp3(CO)3 (the black isomer).

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