Coordinatively unsaturated centres

In addition, an exploration of other aluminium-centred anions, in which interaction with cations can be more subdy varied, has produced a range of low-melting salts and interesting structural types. Part of this is stimulated by an interest in minimising conplexation of metal (and other coordinatively unsaturated) centres by the anionic conponent of ionic liquids used as reaction solvents. The terms innocent and weakly coordinating have been used to describe such minimally interacting species, and the topic has been reviewed [649]. 

Some of these reactions result, essentially, in the oxidative addition of N0" N03 to coordinatively unsaturated metal centres whereas in others ligand replacement by NO+ occurs — this is a favoured route for producing nitroprusside , i.e. nitrosylpentacyanoferrate(II) ... 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

Addition to unsaturated centres (C=0, C=N, C=C) adjacent to the diene can occur in a diastereoselective fashion, and asymmetric synthesis can be carried out if the diene complex is optically active. As Fe(CO)3 coordinates from one face of the unsymmetrically substituted conjugated dienes, the complexes are chiral and can be resolved to the optically active forms 93 and 94, which are used for asymmetric synthesis. The optically active acetyldiene complex 95, obtained by the acetylation of the optically active diene complex 94, reacts diastereoselectively with PhLi to give 96. The optically active tertiary alcohol 97 is obtained by its decomplexation. The enantiomer 100 can be synthesized by the opposite operation namely the benzoylation of 94 to give 98, and subsequent reaction of MeLi gives 99. The enantiomer 100 is obtained by decomplexation [16]. 

As noted above, many transition metal o-organyl complexes are kineti-cally unstable owing to low-energy decomposition routes, which become available if the metal centre is coordinatively unsaturated. The various decomposition routes will be discussed in turn within the wider context of their reactivity. 

For coordinatively unsaturated carbene complexes, nucleophilic attack may occur at the Lewis-acidic metal centre. For 18VE carbene com-... 

The number of positive charges on a Cr atom in an active centre is a subject of some controversy most authors suppose that centres with Cr2+ are more active than those with Cr3+ [196-198] arguments in favour of the opposite view can also be found [199], It appears that even Cr+ [196], Cr4+ and Cr5 + [200] may occur in active centres. In order to interact with the monomer they must, however, always be coordinationally unsaturated. 

---