Cyclic ligands

•	Examine the structure of Refcode entry FEROCE27.

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The metal bonding fragments of organometallic ligands are not restricted to being acyclic. In fact there are many (tens of thousands of examples in the Cambridge Structural Database alone†). The classic example is that of ferrocene which is comprised of two pentahapto cyclopentadienyl ligands bound to an iron(II) ion. That is, two C5H5- ligands are bonding η5to an iron ion, as depicted in the following diagram:

 

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Like some of the above acyclic organometallic ligands, the atoms of the C5H5- ligand are essentially planar. The ferrocene iron ion is situated between two parallel C5H5- ligands, halfway between the two rings and along the perpendicular connecting the ring centroids. These features are highlighted below, and are more clearly illustrated when you manipulate FEROCE27 in Mercury.

 

Ferrocene (refcode FEROCE27) ring centroids are shown in blue

 

•	Ferrocene is properly named bis(η5-cyclopentadienyl)iron(II) and may be represented by the formula: (η5-C5H5)2Fe. The cyclopentadienyl fragment is often abbreviated Cp, i.e. (η5-Cp)2Fe. The η5 coordination mode is so common for Cp that when hapticity is not noted, i.e. Cp2Fe, the η5 is assumed.‡

•	Examine the structures of the next 6entries in the file. (These include refcodes: NOFPON, MULJIM, PEVHUT, FURROZ, ZOZLAB, TPCPCQ.)

•	Excluding any η1- ligands, identify the hapticity of the organometallic ligands and complete the table below. For diagrams, provide standard organic stick diagrams.

 

CSD Refcode	Compound	Uncoordinated Ligand	Hapticity	Name
			η3 (eta-3)
			η4 (eta-4)
MULJIM			η5 (eta-5)	carbonyl (η5-cyclopentadienyl) diiodocolbalt (III)
			η6 (eta-6)
			η7 (eta-7)
			η8 (eta-8)

 

† Over 40,000 structures containing η2- or greater hapticity carbon atom rings ranging in size from three to eight carbon atoms were in the CSD (database version 5.28, 2007 release).

 

‡ Over 15,000 η5--Cp containing structures were in the CSD (database version 5.28, 2007 release).