Metal Complexes Co-ordination Number Six and Higher

In this introduction, some of the highlights of the past eighteen months will be mentioned, and references will be found to other reviews or publications of a general 

One aspect of the co-ordination chemistry of inert metal ions which has not received the attention it fully deserves is the way in which the changed reactivity of coordinated ligands can be used to preparative advantage. Recent examples show how cobalt(iii) complexes can be employed in this way. 

Other examples of template syntheses centred around cobalt(iii) complexes have appeared. The macrocyclic ligand l,4,8,ll-tetra-aza-6,13-dioxacyclotetradecane [dioxacyclam (3)] can be synthesized by a template reaction between formaldehyde (1.8 mol dm- ) and [Co(en)agly]Cla (0.3 mol dm ) in saturated aqueous LiaCOg. 

Condensation of two moles of formaldehyde produces the a-hydroxymethylserinebis-(l,2-diaminoethane)cobalt(m) ion (4), and a further six moles of formaldehyde condense with the two co-ordinated 1,2-diaminoethane ligands of (4) to produce the co-ordinated dioxacyclam (3) in the [Co(dioxacyclam) (a-HOCHa-serine)] ion. The considerable enhancement of the rate of attack of bases at co-ordinated nitriles is well documented, and recently this has been used to preparative advantage in reactions of co-ordinated acrylonitrile with carbanions (Michael addition)  

In an analogous reaction with nitromethane, a 55% yield of the [(NH3)5CoN= CCHaCHaCHaNOa] ion is obtained in 15 s. The reaction of unco-ordinated acrylonitrile with nitromethane under similar conditions is much slower, the half-life of the reaction being greater than 7 h. Surprisingly, it was even possible to reduce acetonitrile co-ordinated to cobalt(in) by sodium borohydride (pH 9 and 298 K) without complete reduction to cobalt(n). A 50% yield of [(NH3)5CoNH2CHaMe] + was obtained in this way.  

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Inert Metal Complexes Co-ordination Numbers Six and Higher... 

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