Coordination unsaturated compounds

Addition reactions to coordinatively unsaturated compounds are expected to be significantly faster than substitution reactions. 

There is considerable evidence that at least many of these reactions require coordinately unsaturated compounds to proceed. In those cases, a dissociation step may be the first part of the reaction. Then the.se coordinately unsaturated compounds react with an unsaturated molecule—it can be most anything as long as it has an available pair of electrons—and this inserting molecule goes in between the metal atom and one of the groups initially bonded to the metal. If the starting material is coordinately unsaturated, so is the product. A final step must be the formation of a coordinately saturated product by some final reaction, either with another ligand or by decomposition of this insertion product. 

Coordinatively unsaturated compounds, whether transition metal or main group element, are capable of accepting electron donor ligands, i.e., they act as Lewis... 

Acceptor Properties or Lewis Acidity of Complexes. Coordinatively unsaturated compounds, whether transition metal or not, can generally add neutral or anionic nucleophiles, e.g.,... 

The first solvent to be adopted at Hanford was methylisobutylketone ("MIBK" or "Hexone"). This solvent forms adduct compounds with coordinatively unsaturated compounds like the actinide nitrates, e.g. Pu(N03)4 2, where S repres ts tihe adduct molecule... 

A limited number of complexes containing coordinated H28 have been prepared, and very few have been structurally characterized.The synthetic methods used involve addition to coordinatively unsaturated compounds, displacement of labile ligands, or protonation of hydrosulfido ligands. Group 6 derivatives ° such as... 

Another type of bridged systems contains the central metal atom in the oxidation state + 3. Appropriate bridging ligands are negatively charged molecules as e.g. cyanide, thiocyanate or azide. The displacement of the axial anion X" by bidentate CN , SCN or NJ (L ) in a coordinatively unsaturated compound PcMX (X = Cl, OAc, CCI3COO) provides a direct path to coordination oligomers [PcML] . This route has been utilized for the synthesis of cyano complexes [PcM(CN)] (M = Fe [116], Mn [117]), thiocyanato complexes [PcM(SCN)] (M = Fe [66], Mn [118]), Co [119], and azido complexes [PcM(N3)] (M = Cr [118], Mn [118]). 

In the coordination unsaturated compounds (which are able to coordinate with one or two ligands) such as the Wilkinson complex, an aldehyde carbonyl, nitrogen group of diazo compound and carbon-carbon unsaturated bond oxidatively add to the compounds, and the cyclization is liable to proceed by reductive elimination [98-102]. For example, 4-pentene-l-al derivative reacts in the presence of the Wilkinson complex to afford prostaglandin derivatives by cyclization [98,101]. When the reaction is performed with a chiral rhodium complex, the cyclization product is obtained in a high ee% yield [102]. The cyclization usually forms five or six-membered ring compounds, the reaction of a diazo compound with acetylenic... 

According to this scheme, hydrogenation is effected by the addition of hydrogen to coordinatively unsaturated compounds with electron deficiency in the d-shell of the metal atom. 

Figure 15.15 schematically shows a cyano-bridged intrinsic semiconductive phthalocyanine complex that can be synthesized by the displacement of the axial anion X by CN" in a coordinatively unsaturated compound PcMX. This synthesis has been used for the preparation of [PcMn(CN)L [184], [PcFe(CN)], [185], and [2,3-NcFe(CN)], [186]. The starting materials were synthesized by oxidative chlorination of the appropriate phthalocyanines with thionyl chloride or oxygen and concentrated aqueous hydrochloric acid. The chlorides PcMCI (M = Fe, Mn) were converted into the bridged complexes [PcM(CN)] in aqueous or ethanolic alkali metal cyanide solutions. 

Coordinatively unsaturated compounds can generally bind neutral or anionic nucleophiles. Such reactions are called additions. Oxidative additions are reactions that involve an increase of the formal oxidation state and the coordination number. Reductive eliminations are, in principle, opposite processes to oxidative additions. They lead to a reduction of the coordination sphere of the metal and to its formal oxidation state. 

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