Coordinatively unsaturated intermediate

The first step consists in the attack of a proton on the W-H bond to yield a labile dihydrogen intermediate (Eq. (3)) that rapidly releases H2 to form a coordi-natively unsaturated complex (Eq. (4)). This complex adds water in the next step to form an aqua complex (Eq. (5)) that completes the reaction by substituting the coordinated water by the X anion (Eq. (6)). Steps (3)-(6) are repeated for each W-H bond and the factor of 3 in the rate constants appears as a consequence of the statistical kinetics at the three metal centers. The rate constants for both the initial attack by the acid (ki) and water attack to the coordinatively unsaturated intermediate (k2) are faster in the sulfur complex, whereas the substitution of coordinated water (k3) is faster for the selenium compound. 

At high PPh3 concentrations, where the catalyst resting state is (PPh3)3Rh(CO)H, phosphine dissociation must occur to form the coordinatively unsaturated intermediates 3c and 3t. This dissociation is suppressed by increased PPh3 concentration, which serves to reduce the concentration of active Rh species in the catalytic cycle. 

The kinetic data reveal a complex dependence on the anion concentration and the hydrogen-ion concentration and have been interpreted on the basis of ion-pair and ion-triplet formation. The uncatalyzed path ( 0)has been shown to involve (NH3)5Co(OH)Co(NH3)55+ (= M5+)and the ion pair M x Y4 +, and it was proposed that the ion pair M x Y4+ scavenges Y- from solution and not from the second coordination sphere (357). It was shown that the reactive intermediates are quite selective for anions (as well as being selective for the N terminus of NCS-, the ratio for N-bound S-bound being approximately 4), and this has been interpreted as arising from a genuine, coordinately unsaturated intermediate. The acid-catalyzed path has been interpreted in terms of the formation of protonated unaggregated reactant, MH6+, and small concentrations of the protonated ion pairs and ion triplets MH x Y5+ and MH x Y24+ (355, 356). 

Carbon dioxide insertion into the W-C bond of CH3W(CO)j was not retarded by excess carbon monoxide. In other words, as Fig. 10 illustrates, the C02 insertion process does not involve a coordinatively unsaturated intermediate. This observation could only be made when an alkali metal counterion was present, since the rate of C02 insertion was much faster than that of CO insertion under this condition. On the other hand, [PNP][CH3W(CO)5] undergoes CO insertion (80) at a much faster rate than carbon dioxide insertion. Both processes exhibited similar metal (W > Cr) and R(CH3 > C6H5) dependences. 

Photoinduced formation of binuclear tetrapyrroles (in all instances polynuclear means binuclear in this Section) is a rare process. In reality, it is an addition reaction involving two ground-state species, at least one of them being a coordinatively unsaturated intermediate formed in the primary photochemical step. A true photochemical binuclear complex formation, involving an electronically excited complex with enhanced Lewis acidity, has not been observed so far (the formation of excimers - Sect. 2 - does not fall in the category of polynuclear complex formation as excimers are excited-state species). 

Both Marko and Aldridge postulate tricarbonyl intermediates to account for the inverse dependence of rate on carbon monoxide pressure. Reaction of such intermediates with molecular hydrogen as in Eq. (61) is discussed in the section on coordinatively unsaturated intermediates. 

In both Re(CO)5X and M(CO)s(amine) higher energy photolysis leads to population of LF states which feature population of the dxzy2 bi )° y orbital which labilizes the equatorial CO s and leads to larger CO substitution quantum yields. In all of these C4v complexes the ligand photosubstitution most likely occurs by strictly a dissociative mechanism to yield coordinatively unsaturated intermediates. For the Re(CO)sX, photolysis in the absence of added nucleophiles yields the dimeric species [Re(CO)4X]2 reaction (12), which likely form via coupling of two coordinatively unsaturated Re(CO)4X intermediates.68 ... 

Photodissociation of coordinated ligands has also lead to the synthesis of new complexes by oxidative addition to the coordinatively unsaturated intermediates as in reactions (17)—(19).85-87 These types of reactions have been invoked in transition metal complex photoassisted and photocatalyzed reactions. 

The M(CO)6 (M = Mo, W) photoassisted interconversion of the linear pentenes, reaction (51), is an example of a situation where the role of the light, at least in part, is to generate a reactive intermediate which is responsible for the isomerization reactions.137 The key photoreaction is dissociative loss of CO from W(CO)s(alkene), reaction (52), to yield a coordinatively unsaturated intermediate which can lead to... 

A major supposition of this computational mechanistic study is the separation of the photochemical and thermal reaction events. It has been assumed, Scheme 1, that a photochemical reaction takes place to generate a coordinatively unsaturated intermediate that subsequently reacts thermally with dihydrogen. In other words, we are assuming that the reaction is photochemically initiated but that light plays no role in later steps of the reaction (for at least one cycle). 

The observation of inhibition of the H/D exchange reaction by CO and CH3CN implicates coordinatively unsaturated intermediates in the H2 capture process. As a part of this manuscript we report evidence for the existence of an acetone derivative of the (p-H)(Fen)2 complex, as suggested by theory. As it has obvious ramifications for technical development of such H2-uptake catalysts, the possibility that water might similarly compete for the open site and serve both as a required reagent and a catalyst inhibitor deserves a future detailed study. 

The stereochemical consequences of L for X- exchange at the ruthenium center have been studied by Morandini et al. (48). Using the pure epimers 34 and 35, it was determined that the exchange of chloride for acetonitrile proceeds stereospecifically with retention of configuration at ruthenium [Eq. (46)] (48). This study provides good evidence that the stereochemical integrity of the ruthenium center is maintained in the coordinatively unsaturated intermediate formed on loss of Cl-. 

The preparation and interconversion of two isomeric iridium(III) trihydrides, fac- and mer-[Ir(H)3(CO)(PPh3)2] (180), have been studied. Under a hydrogen atmosphere, both isomers undergo spontaneous interconversion to a dynamic equilibrium. The kinetic data obtained from the interconversion and H2 displacement from both isomers by PPh3 suggest that the interconversion process occurs via a reversible reductive elimination-oxidation sequence. Both processes are believed to involve the intermediate species [Ir(H)(CO)(PPh3)2] (181). It has further been postulated that the interconversion process proceeds by slow unimolecular loss of H2 (reductive elimination), followed by a rapid readdition of H2 to the coordinatively unsaturated intermediate species (181), as depicted in Scheme 21.423... 

Barefield et al.m report that [Ir(H)5(ER3)2] (ER3 = PPhEt2, PEt3, PMe3) catalyzes the exchange between D2 and benzene, probably via oxidative addition of the C—H bond to a coordinately unsaturated intermediate species. Also, Vaska s complex catalyzes the exchange between D2 and C2H4, concomitantly with hydrogenation.500... 

Interestingly this type of complex is stereochemically rigid at around room temperature but undergoes an intramolecular exchange of PF3 on warming. Such a process suggested the formation of a coordinatively unsaturated intermediate by phosphine dissociation (Scheme 5). Support for this comes from the synthesis of [Rh2(PF3)5( BuCC Bu)], which reacts reversibly with PF3 at room temperature to form [Rh2(PF 3)6( BuCC Bu)]. ... 

The general kinetic scheme shown in Scheme 8 has been developed for alkyl migration in [Mn(R)(CO)s] compounds. Thus, two pathways to the final product are avaUable (i) a second-order pathway (AiMn(R)(CO)5][L]) and (ii) a two-step sequence via the coordinatively unsaturated intermediate [Mn(COR)(CO)4]. In polar solvents, this intermediate may exist as a solvated hexacoordinate species, [Mn(COR)-(CO)4(S)] (S = molecule of solvent). In the absence of a suitably polar solvent, the intermediate is thought to adopt a square-based pyramidal conformation (99,100) with the acetyl ligand occupying a basal position, although a 7r-acyl derivative has also been proposed (101,102). 

---