Hydride-carbonyl complexes

Two hydrogen bridges are present in some carbonyl hydride complexes. This is illustrated by the structure of the [H2W2(CO)8]2 anion,... 

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

In this chapter we have examined examples of polynuclear metal carbonyl complexes as well as simple metal carbonyl hydrides. Consider now the polynuclear carbonyl hydride complex, H,Os,(CO)i2- Rationalize the formulation of this species. From your application of the 18-electron rule, what can you say about the structure of this molecule1 How is it similar to or different from the complex Os COln shown in Figure 5.9 (See Churchill M. R. Wasserman, H. J. Iticrg, Chen. 1980, 19, 2391-2395.)... 

Although in m iny respects carbonyl hydride complexes mav he regarded as acids, they also show sonic similarities lo the basic hvdndcs of the main gioup metals te.g.. 

Alkylcobalt carbonyl isomerization via the formation of olefincobalt-carbonyl hydride complexes with retention of the olefin attached to the cobalt atom has been suggested as the mechanism of formation, of the necessary precursors of the products with high stereospecificity. 

Hydrolysis of (14) readily yields MeOH (78). The carbonyl hydride complex (13) forms by the addition of CO to Zr(C5Me5)2H2 and possibly by the reaction of Zr(C5Me5)2(CO)2 with H2, although only the former route allows its observation. In the absence of the dihydride complex Zr(C5Me5)2H2, (13) reacts with itself on warming from -80°C to 25°C yielding the unusual enediolate dimer (15) with the structure postulated as shown (78). 

The tris(triphenylphosphine) rhodium carbonyl hydride complex also was used via ligand exchange to obtain known chelate complexes of bisphosphines... 

Since there were no known definitive studies of the crucial equilibria between the tris-(triphenylphosphine)- and bis-(tri-phenylphosphine)rhodium(I) carbonyl hydride complexes, it was decided to study such equilibria by P-31 NMR spectroscopy. The... 

Since the 1-olefin concentration-dependent hydroformylation in the presence of the above catalyst system has a slightly higher activation energy of about 22 kcal mol-1, it is proposed that the ratedetermining step of selective terminal 1-olefin hydroformylation may involve a transition state leading to the formation of a 1-alkyl bis-(trans-phosphine)rhodium carbonyl hydride complex rather than the dissociation of the trisphosphine complex. 

Figure 2.13 Synthesis of iron carbonyl hydride complexes...

Carbonyl hydride complexes can also be formed by the reduction of carbonyl complexes, followed by the addition of acid. For example. 

Although in many respects carbonyl hydride complexes max be regarded as acids, they also shoxv some similarities to the basic hydrides of Ihe main group metals le.g.. 

In some cases a novel reverse carbonylation reaction may also occur, affording carbonyl hydride complexes, for example ... 

The treatment of many of the metal carbonyls with bases affords complex polynuclear carbonylates (see (140) for a recent review). As is the case for the mononuclear carbonylate anions, hydrolysis of the anions, often with dilute acids, affords diamagnetic metal carbonyl hydride complexes (see Table VI). The majority of these complexes which are described below have not been studied by infrared and proton magnetic resonance spectroscopy their formulations are based primarily on analysis and mode of decomposition, and thus remain tentative. 

Catalytic Reactions Involving Cobalt Carbonyl Hydride Complexes... 

Perhaps the most important development in polynuclear carbonyl hydride complexes was the observation by Kaesz et al. that good yields of hydridocarbonyls could be obtained by direct reaction of hydrogen with polynuclear carbonyls (82). This has been applied to a number of systems. Thus, the anion [Rh,2(00)39 yields the cluster anion... 

The NMR (Fig. 5) indicates the presence of three major hydride species, two of which result from fragmentation of the [Rh2(rac-et,ph-P4)] carbonyl and carbonyl-hydride complexes. The temperature-independent hydride resonances at... 

Brown has shown that metal-carbonyl-hydride complexes such as [Re(H)(CO)s] can nndergo CO substitntion by a phosphine (PBus) according to the H-atom-transfer-chain mechanism A classic type of initiation to introduce the radical species into the chain is to photolyze the metal-carbonyl dimer, which generates the reactive 17-electron metal-carbonyl monomer ... 

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