Metallic bond unsaturated behaviour

A metal ion might play a number of roles in modifying the behaviour of co-ordinated electrophiles. Co-ordination of a donor atom X to the metal polarises the C-X bond and increases the electrophilic character of the carbon. In the case of an unsaturated centre this polarisation may be countered or even completely reversed by back-donation from filled (d) orbitals on the metal into vacant 7t -(or similar) orbitals of the ligand. The X- group generated after nucleophilic attack on the carbon atom may also be stabilised by co-ordi-... 

This behaviour correponds to the observation that other unsaturated hydrocarbons, e.g. alkynes, allenes or 1,3-butadienes, which readily undergo transition metal catalyzed cyclooligomerizations, do also incorporate CX multiple bonds in such cycloadditions only with difficulty in most cases 207 208). Besides the well known cobalt-catalyzed pyridin synthesis from alkynes and nitriles98 cocyclooligomerizations have been achieved with alkynes on one side and isocyanates 209), carbodiimides210) and carbondioxide 211) on the other side as well as with 1,3-butadienes and aldehydes 212), carbondioxide213 and 2-aza- or 2,3-diaza- 1,3-butadiene214. ... 

Sr " 118, Ba + 135 pm) when compared to Mg " " (72 pm) — i.e the lower (charge/size) ratio enhances. still further the ionic character of the bonding and thus increases the kinetic lability of the ligands. Coordinative unsaturation also plays a r61e and, indeed, the organometallic behaviour of the heavier alkali metals often resembles... 

This behaviour correponds to the observation that other unsaturated hydrocarbons, e.g. alkynes, allenes or 1,3-butadienes, which readily undergo transition metal catalyzed cyclooligomerizations, do also incorporate CX multiple bonds in such cycloadditions only with difficulty in most cases Besides the well known cobalt-... 

This work also gave an insight into the electrochemical behaviour of complexes with multiple metal-carbon bonds (presenting aminocarbyne, carbyne, T] vinyl, vinylidene or carbene ligands), a field which has not yet been explored in spite of the rich chemical reactivity which has already been developed for such a t e of species. Further developments are also expected, particularly in the fields of the electroactivation of the C- or N-unsaturated species and of the mechanistic investigation of their redox processes. 

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