Organometallic Aquo-ions

The features of organometaUic aquo-ions are two-fold first they display a hemisphere (usually one face of the coordination octahedron) that is shielded against substitution, thus, invariant and second they possess a number of water ligands that can be substituted. The positions occupied by water represent the 

Although we may become excited about the properties of organometallic compounds, we should not ignore the limitations imposed by the invariant portion of the complex. Desirably, the facially coordinating ligands should be a CgH or a [CsHs] ligand since neither is sterically demanding and they are of low molecular 

Clearly, a more convenient way is required. Beside the usual reducing agents, boranes have infrequently been used in this respect with transition elements although they are easy to handle and very oxophilic. We reacted [ TcOJ with BHj-THF under 1 atm of CO and in the presence of Cl as a potential counterion. At room temperature and after a relatively short time period, the complex [ TcCl3(CO)3] could be isolated in good yield [29, 30]. The reaction pathway is unclear since no intermediates could be authenticated. With the exception of minor amounts of [TcH(CO)4]3 no higher carbonyl complexes were found or isolated [30]. The Jac-[Tc(CO)3] moiety is thermodynamically the most stable unit, since any additional CO trans to another CO is labile and readily cleaved. The reaction mechanism at this point still remains unclear since the CO concentration in solution requires delicate control and other complexes might be present as well to explain the reaction pathway. 

This nature is the essential base for the introduction of [ Tc(OH2)3(CO)3 in bioorganometallic chemistry. The different points outlined earher are perfectly fulfilled to enable radiopharmaceutical drug finding based on the ac-f c(CO)3j moiety. Three of the coordination sites are blocked by the CO ligands and three coordination sites are available for coordination to a wide variety of ligands and chelators. 

In that respect, [ Tc(OH2)3(CO)3 resembles a true aquo-ion and compares with e.g. the lanthanides where H2O ligands also have to be substituted to form the radiopharmaceutical. 

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M = Os (the organometallic aquo ions were enriched previously in H2170). 

Scheme 4.5 General reactivity patterns of organometallic aquo- (or solvent) ions leading to novel bioorganometallic compounds and/or (radio)pharmaceuticals [15, 25].

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