Organometallic aqua ions

Although there is a tendency to associate coordinated water with Werner-type complexes, where it is extensively established, organometallic aqua ions are known.945 The simple [(Cp )Co(OH2)3]2+ has been established, and is prepared via Equation (8). The lower pATa is 5.9, similar to values in aminecobalt(III) compounds, and reversible deprotonation and dimerization has been identified as part of the reactions of the aqua ion.946... 

Enormous acceleration of substitution at ruthenium(II) can be obtained by appropriate choice of ligands. Following the demonstration of remarkably rapid water exchange and complex formation (7d or D in mechanism) at the organometallic aqua-ion [Ru(ri5-C5Me5)(H20)3]2+ (150), comparably rapid substitution has been demonstrated at... 

Scheme 2 Examples of organometallic aqua ions that are currently under investigation for application in life science...

BC is reasonably stable at neutral-to-alkaline pH but decomposes rapidly in acidic media. The reaction of [99mTc04]" with BC at pH 11-12 yields quantitatively the organometallic aqua ion [99mTc(OH2)3(CO)3]+. Mechanistically, it is possible that BC acts as a ligand which binds to the technetium centre. Hydride transfer followed or paralleled by reduction occurs concomitantly with CO coordination. The X-ray structure of a model with K+ as the metal is shown in Fig. 1. 

Table 1 Rate constants and activation parameters for water exchange on organometallic aqua-ions and related hexaaqua-ions...

Although the preponderance of recent reports centres upon some inner transition metal ions (lanthanides), one prominent investigation features water exchange on a first row organo-transition metal compound. Organometallic aqua ions in which a transition metal ion... 

For organometallic systems, acidity values for a number of metal carbonyl aqua complexes have now been reported. In some cases, these complejKS have enhanced rates for water exchange relative to the hexaaqua ion of the same metal. Ease of water exchange in organometallic aqua ions of technetium is essential toward the generation of " Tc-labeled biomolecules for radiopharmaceutical imaging applications. 

Oxidation A wide range of +3 compounds is formed as well as aqua ions. The ionic radius decreases states +3 gradually across the series, leading to changes in solid structures, and an increase in stability of complexes in solution. Organometallic compounds are more ionic than in the d block. 

Transition metal ions can bind ligands (L) to give a coordination compound, or complex ML , as in the familiar aqua ions [M(OH2)6] (M = V, Cr, Mn, Fe, Co, or Ni). Organometallic chemistry is a subfield of coordination chemistry in which the complex contains an M—C or M—H bond [e.g., Mo(CO)6]. Organometallic species tend to be more covalent, and the metal is often more reduced, than in other coordination compounds. Typical ligands that usually bind to metals in their lower oxidation states are CO, alkenes, and arenes, for example, Mo(CO)g, (C6H6)Cr(CO)3, or Pt(C2H4)3-... 

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