Between Coordinatively Unsaturated Species

The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

It is meaningless trying to find a relation between the nature of excited states and the ability of complexes to undergo photodimerization, since in the corresponding addition reactions ground-state molecules participate. The formation of coordinatively unsaturated species should occur from the excited states involving the central atom as one of the central atom - axial ligand bonds must be cleaved. 

Iron pentacarbonyl exhibits efficient photodecomposition (in the absence of ligands) because the bimolecular reaction between Fe(CO)3 and photogenerated Fe(C0)4 yields insoluble Fe2(C0)g. Sterically unhindered Fe(CO)3[l,4-Me2N4] mimics the behavior of Fe(CO)3 in forming a cluster on irradiation in the absence of ligands. Furthermore selective photodissociation of CO from the tetraazabutadiene complex produces a coordinatively unsaturated species that reacts (like photogenerated Fe(C0)4) with Fe(C0)3 to form a dimer, Equation 10. 

On the basis of mechanism, condensation processes can be divided into two broad categories (a) reactions induced by coordinatively unsaturated species and (b) reactions between coordinatively saturated species in different oxidation state (redox condensation). 

The intimate mechanism of the reaction deserves special attention not only because it was the first example of alkane activation by a metal complex. Activation of alkanes by platinum(II) complexes remains unique in many aspects. The reaction takes place in neutral water solution with conventional chloride ligands at the metal without special ways to form coordinatively unsaturated species (e.g., by irradiation). A number of works were directed towards the elucidation of the nature of the interaction between an alkane and a platinum(II) complex. The unique feature of platinum(II) complexes is to exhibit both nucleophilic and electrophilic properties. 

Scheme 24.40 implies that the intermediate is a coordinatively unsaturated species. In the presence of a solvent, S, such a species would probably be stabilized as Mn(CO)4(COMe)(S). In the absence of solvent, a 5-coordinate intermediate is likely to be stereochemically non-rigid (see Figure 3.13 and discussion) and this is inconsistent with the observation of a selective cis-relationship between the incoming CO and acyl group. It has been concluded from the results of theoretical studies that the intermediate is stabilized by an agostic Mn—H—C interaction (structure 24.46), the presence of which locks the stereochemistry of the system. ... 

To measure the relative rates of various ligands to replace CO, chemists must turn to competition experiments (see Section 8.8.3). Competition experiments using the coordi-nately unsaturated 16-electron species Ni(CO)a, Fe(CO)4, and Mo(CO)s, and alkenes, phosphines, and amines as ligands show only small differences in product ratios (between 1 and 10). Because these three 16-electron coordinately unsaturated species are not very selective in their reactions, they must all be very reactive. They react with almost any nucleophile with a very low energy of activation. In support of this view, the rate constants for the addition of the new ligands to such unsaturated systems have been estimated to be near 10 M s that is, they are approaching diffusion control. 

Kinetic studies of this process do indeed show the catalysis being first order in the concentration of the ruthenium cluster and inversely dependent on the partial pressure of carbon monoxide indicating thus that the most important intermediate in the reaction is the coordinately unsaturated species H4Ru4(CO)ii. Coordinately unsaturated sites in the catalyst would be achieved, in this case, by the dissociation of a CO-ligand. The catalytic effect of the unsaturated intermediate depends on the reversible insertion of ethylene into the Ru-H bond. However there is competition between this insertion and the addition of carbon monoxide or H2 for obtaining the starting compound or the hexahydride H6Ru4(CO)n respectively. 

Knox and co-workers synthesized a triruthenium cluster by the reaction of the coordinatively unsaturatcd diruthenium alkyne complex (CpRu)2(/r-CO)(/r-RCCR) 39 with a monometallic carbonyl complex M(CO)4(L) (M = Fe, Ru). Two isomers, 40 and 41, were formed in the reaction of 39 with Ru(CO)4(CH2 = CH2) (Equation (13)). The ratio between 40 and 41 was shown to be dependent on the nature of the substituents of the alkyne. In the case of diphenylacetylene complex, coalescence of the H signals of these isomers, 40a and 41a, was observed. This shows that isomerization between the two isomers took place at considerable rate. In contrast, the reaction of 39 with Fe(CO)4(thf) exclusively afforded a 3- ( )-alkyne complex, in which the alkyne moiety was 7r-coordinated to an iron center. Knox and co-workers also reported the syntheses of triruthenium /i3-alkylidyne complexes by the photolysis of a bimetallic /r-alkylidene complex and a bimetallic diruthenacy-clopentenone complex. In these reactions, formation of the triruthenium frameworks was rationalized by the coupling reaction of the monometallic coordinatively unsaturated species generated by the photolysis with the starting bimetallic complexes. 

Finally, the active sites for the hydrogenolysis of asym DAM are Mo(IV) species that originated from the reduction of the octahedral Mo(VI) species. The adsorption of the aryl group occurs on the coordinatively unsaturated molybdenum sites, which have acidic properties this fact, in turn, leads to the reaction mechanism of the interaction between the active species and the substrates. 

A route for designing Gd(HI) complexes whose relaxivity depends on the presence of lactate, is provided by the ability shown by some hexa- or hepta-coordinate chelates to form ternary complexes with a wide array of anionic species (154-161). The interaction between the coordinatively unsatured metal complex and lactate involves the displacement of two water molecules coordinated to Gd(III) ion with the two donor atoms of the substrate, thus leading to a marked decrease in the relaxivity. Lactate is a good ligand for Gd(IH) ion because it can form a stable 5-membered ring by using the hydroxo and carboxylic oxygen donor atoms (Fig. 19). 

The distinction between coordination and organometallic chemistry, implicit in the title of this series, is not clear cut for hydride complexes because so many coordination complexes of the hydride ligand either react with unsaturated organic compounds to give, or are formed from, organometallic species. We shall therefore cover all aspects of hydrides, while emphasizing coordination chemistry. 

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