Coordinatively unsaturated complexes

Tilley and co-workers reported the isolation of coordinatively unsaturated complexes Cp Ru(L)Cl (Cp =/j -C-sMes, L = PCy3 and P Pr ) by a simple reaction with the tetrameric species fCp RuCI]4. This synthetic pathway has allowed us... 

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

Treatment of (239), R — Me, with KO Bu in toluene results in formation of the coordinatively unsaturated complex [Ir=CH2 N(SiMe2CH2PPh2)2 ], (241). A summary of the reactions of (241) is given in Reaction Scheme 22. Possible mechanisms have been probed using NMR spectroscopy. 

The only Zr(0) carbonyl complex has been prepared by Wreford and co-workers. When ZrCl4 was treated with l,2-bis(dimethyl-phosphino)ethane (dmpe), and then subsequently reduced with sodium amalgam in the presence of 1,3-butadiene, the dmpe bridged dimer, [(t/-C4H6)2Zr(dmpe)]2(dmpe) (65), resulted (114). The brown crystalline dimer 65 was found to be in equilibrium with the 16-e- coordinatively unsaturated complex, (tj-C4H6)2Zr(dmpe) (66), and free dmpe. When toluene solutions of 65 were exposed to CO at —45°C, 1 equivalent of CO per equivalent of Zr was consumed and the CO adduct (r/-C4H6)2Zr-(dmpe)(CO) (67) precipitated as a yellow solid. If these mixtures were allowed to warm above -22°C under vacuum, the precipitate dissolved and the consumed CO evolved (114). Complex 67 could be isolated by... 

High-pressure photochemistry has been used very successfully in studying the mechanisms of catalytic reactions. Irradiation of a suitable precursor permits in-situ preparation of reactive intermediates such as coordinatively unsaturated complexes or radicals. It is thus possible to check whether these species are involved in the catalytic cycle. 

The encapsulation of reactive organometallic complexes is not restricted to the anionic [Ga4(L13)6]12- cages. Thus, Fujita and coworkers were able to generate the coordinatively unsaturated complex [Cp Mn(CO)2] (Cp = C5H4Me) within a self-assembled [M Le] coordination cage 28 (Fig. 20) (132). Photoirradiation of solid 27 gave complex 28, the crystal structure of which confirms the presence of the unsaturated pyramidal [CpMn(CO)2] fragment. The direct observation of such intermediates is... 

Insertion of COz into a metal-hydride bond normally requires the prior dissociation of an ancillary ligand to generate a coordinatively unsaturated complex, because C02 coordination to the metal usually precedes the formal insertion (Scheme 17.3, lower pathway). Ah initio calculations [59] support this mechanism for the insertion of C02 into the Ru-H bond of RuH2(PH3)4, a model for the catalyst RuH2(PMe3)4. However, it is theoretically possible for C02 insertion to take place without prior C02 coordination (Scheme 17.3, upper pathway) [60, 61]. The... 

Coordinatively unsaturated complexes and those giving easily such species by ligand dissociation favor pathways related to that described in Eqs. (10) and (13). Coordinatively saturated complexes reduce halocarbons via outer-sphere ET [193, 194]. In cases of electrochemical dehalogenations, the species formed by one-electron reduction of the mediators on the cathode often react in this way [156, 157, 198], For example (Eq. (14)) [157, 166] ... 

The reactions of coordinately unsaturated complex ions using a combination of ES and CID may be a rich area of gas-phase coordination chemistry in the future. The diethylenetriamine (dien) platinum(II) complexes have been investigated using this method (194). 

The existence of the coordinatively unsaturated complex C also rationalizes the sensitivity of the catalyst towards weakly coordinating species such as N2 and heterocycles, as well as towards substrates containing more labile C—H bonds, such... 

However, even this simplified formula does not justify the use of the ratio of stability constants of the extracted complexes as the only measure of selectivity of extractive separations. Such a widely used approach is obviously based on an implicit assumption that the partition constants of neutral complexes ML of similar metal ions are similar, so that their ratio should be close to unity. This is, however, an oversimplification because we have shown that the ifoM values significantly differ even in a series of coordi-natively saturated complexes of similar metals [92,93]. Still stronger differences in the values have been observed in the series of lanthanide acetylacetonates, due to different inner-sphere hydration of the complexes (shown earlier), but in this case, self-adduct formation acts in the opposite direction [100,101] and partly compensates the effect of the differences in. Tdm on S T(see also Fig. 4.15). Such compensation should also be observed in extraction systems containing coordinatively unsaturated complexes and a neutral lipophilic coextractant (synergist). 

A systematic examination of the stoichiometric and catalytic chemistry of both of these coordinatively unsaturated complexes is being undertaken. The following are some representative reactions (16, 81, 134) 2... 

Treatment of [TaCl2(dmpe)2] with two equivalents of vitride offered [TaCl(CO)2(dmpe)2] (53) under CO, while a coordinatively unsaturated complex, [Ta H2Al(OC2H4OMe)2 (dmpe)2]... 

It might be possible to make a more active catalyst (per mole complex dissolved per unit volume of solvent) either by increasing the tendency towards the formation of coordinatively unsaturated complexes in solution or by increasing the rate constant of the rate determining step, i.e., the oxidative addition of hydrogen. 

This makes it easy to predict that solutions of HRh(PPh3)4 are more active catalysts than solutions of HRh(CO) (PPh3)3, either because the concentration of coordinatively unsaturated complexes increases or because the rate constant of the oxidative addition of hydrogen increases. A kinetic investigation of the hydrogenation of hexene catalyzed by solutions of hydridotetrakis( triphenylphosphine) rhodium (I) is reported here. 

The kinetics of this reaction have been studied in detail and a hydroxy-carbonyl is specifically proposed as an intermediate consistent with the kinetic data. Decomposition of this intermediate hydroxycarbonyl may proceed by -elimination of the platinum hydride product since the hydroxycarbonyl is a 16-electron coordinatively unsaturated complex. Another well-known example of metal hydride formation from CO and H20 is the reaction of iron carbonyl in aqueous alkali (55) (36). 

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