Structure, Stereochemistry, and Spectra

Structural aspects of heterocyclic systems that contain a metal heteroatom have been little explored. Such a wealth of data is available for carbocycles that these will be used as initial structural models for the organometallic systems. The questions related to the effect of replacing a —CHg— group in a carbocycle by a metal atom are many-sided. In the next few paragraphs the structures of carbocycles and their benzene-annelated derivatives will be briefly reviewed, followed by a discussion of the problems introduced, or changes that may be expected, upon heteroatom replacement. Then the data observed for organometallic systems will be discussed in terms of the points raised in these sections. 

The larger the ring size the greater the number of ring conformers that must be considered. Benzcarbocycles may exist either in chair or boat forms. The following are some possible forms of benzcycloheptane  

Additional multiple bonds in the benzcarbocycle tend to favor the boat or twisted boat form. 

Dibenzcarbocycles are of two general types, i.e., derivatives of biphenyl or those in which the benzene rings are separated by one or more —CH2— groups. These two structural types are shown below for a system with a central six-membered ring  

Tricyclic systems with a central ring size of six or seven members 

---