Cyclopentadienyl complexes classification

Metallocenes with substituted cyclopentadienyl rings. Metallocenes with methylated rings were among the first heavy alkaline earth metallocenes to be structurally characterized, but many other substituents have been incorporated into bis(cyclopentadienyl) complexes. Under this classification are included compounds with indenyl ligands, which in... 

Similarly cyclopentadienyl complexes have been treated as being formed by the radical CjHs. In other words, they are considered as acting as 4- or 5-electron ligands respectively when they bond to a transition metal. This classification is really a formality, and the complexes could just as well be thought of as being derived from the 6-electron aromatic anions 4114 and CsHs" respectively and a metal cation carrying a suitable positive charge (-1-2 or -1-1) to maintain electroneutrality. The total number of electrons in the complex, of course, remains the same. 

The mass spectra of a considerable number of 77--bonded complexes of the group VIA metals have been reported, but in many cases mass spectrometry has only been used to determine the molecular weight, so that a detailed examination of the fragmentation processes involved has not been attempted, and only the molecular ion and perhaps a few other major peaks are reported. Within this section it is more convenient to discuss the compounds in terms of the attached ligands rather than in terms of the central metal atom. The classifications are (A) cyclopentadienyl compounds (B) arene compounds and (C) olefin, acetylene, and allyl compounds. 

These complexes barely deserve a separate classification in that their chemistry is very similar to that of the simple carbonyl hydrides. All are diamagnetic and their formulation obeys the rare gas rule. Only rarely does the jr-cyclopentadienyl group play a distinctive role in the chemistry of these complexes, and for the majority of the chemistry it may be regarded as an inert ir-bonding ligand. 

All of these three classes can produce bridge-terminal and/or terminal-terminal exchange, depending upon the particular example in question. In comparison with binuclear complexes, the dissociation pathway of [( -C5H5)Cr(CO)3]2 has not been identified for any cluster. Since most of the dinuclear complexes studied contain cyclopentadienyl ligands which, whenever evidence can determine this, stay bound to one metal, it seems more appropriate to place their exchange processes in the third classification. 

The classification of metal 7r-complexes has customarily been based on the types of organic 7c-ligands rather than on the metal elements in the TT-complex. In accordance with this classification, the three major groups of olefin, Ti-cyclopentadienyl, and arene metal Tc-complexes as well as their subgroups are described in the following by illustration of typical 7c-complexes. 

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