Hydrido complex intermediate

Hydrido complexes of all three elements, and covering a range of formal oxidation states, are important because of their roles in homogeneous catalysis either as the catalysts themselves or as intermediates in the catalytic cycles. 

Tetraphenylmolybdenocene dihydride Mo(r 5-C5HPh4)CpH2 (45) was formed by addition of diphenylacetylene to MoCpL(PhC CPh)CH3 (L = P(OMe)3) (Eq. 15), presumably via an ot-hydrogen abstraction to an intermediate methylidene hydrido complex, followed by addition of two equivalents of diphenylacetylene and C — H insertion with concomitant elimination of L [57 b],... 

The proposed mechanisms are similar in both cases and involve in particular an (aryl)(hydrido)ruthenium intermediate in which the ruthenium is additionally coordinated by an in. ( ////-generated /V-phenylimine moiety tethered to the same Ru-bound aromatic ring. The C-C bond-forming step for the construction of the corresponding heterocyclic framework proceeds via insertion of the C=N double bond into the C-Ru bond with transfer of the (hydrido) ruthenium complex to the now phenylamine nitrogen. The desired heterocycles 158 and 159 were obtained after successive reductive elimination, deamination, and dehydrogenation. 

The monosubstituted adduct offers the ready synthesis of a whole range of monosubstituted adducts (see Scheme 6) it is often possible to isolate in these reactions intermediates that are not readily obtained by alternative methods. Thus, in the reaction with halogen acids to yield the bridged hydrido complexes HOs3(CO)10X, it is possible to identify the intermediate HOs3(CO)uX complex in which the halogen functions as a one-electron donor bonding to only one metal center (158). 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

Another theoretical study also showed that the third pathway (bl +b3+b4), 1,3 hydrogen shift, through a hydrido-alkynyl intermediate could compete with the 1,2 hydrogen shift pathway (bl+b2) when the metal center is electron-rich enough [29, 30]. Indeed several hydrido-alkynyl intermediates have been detected or even isolated during the q -l-alkyne-to-vinylidene rearrangement on electron-rich metal centers, such as Co(I), Rh(I) and Ir(I) [73-78]. The ab initio M P2 calculations by Wakatsuki, Koga and their coworkers on the transformation of the model complex RhCl(PH3)2(HC=CH) to the vinylidene form RhCl(PH3)2(C=CH2) indicated that the transformation proceeded via the oxidative addition intermediate RhCl(PH3)2(H) (C CH) [30]. 

This reaction is believed to proceed through a hydrido amido intermediate [tra/ ,s-Pt(H)(NH2)(PPh3)2l which eliminates PPh3 to form the dimeric [ Pt(H)(p-NH2) (PPh3) 2]. The use of NEt, is sufficient to deprotonate a rhenium complex, as in Equation... 

Addition of excess base in these reactions generates the dianionic starting material in Eq. 15.16. The intermediate shown in Eq. 15.19, which has a coordinated formyl group (C(O)H), has been isolated and characterized. Its counterpart in the OH reaction is a hydroxycarbonyl complex (see Problem 15.45) Both are inherently unstable, decomposing to the hydrido complex by elimination of CO or CO,. When these reactions are attempted with Cr(CO)fc, the final product is not the expected [HCriCO) ] anion, but rather a bridging hydride complex "... 

Similarly, the thioaldehyde complexes 66a,b were formed via addition of thiols to the vinylidene complex 64 and elimination of ethene. The vinylidene complex 64 reacted with benzyl mercaptan to yield the isolable intermediate 65. On heating 65 lost ethene in a first-order reaction to give 66b.188 The complexes 66a-d could also be prepared from the benzyne-(hydrido) complex 67 and thiols RCH2SH (Scheme 18).188... 

Unexpectedly, the methoxo complex shows no tendency to decompose to a hydride by / -elimination, although it does react immediately with water to give the hydroxo complex. In contrast, cyclohexyne bis(triphenylphosphine)platinum(0) reacts with methanol to give a hydrido complex, presumably via an undetected intermediate methoxide (see Equation 3) ... 

The formation of the unsaturated intermediate Run(OEP) from the hydrido-complex RumH(OEP) is supposed to occur from a LMCT excited state. As a consequence, the dimeric [RuH(OEP)]2 with a metal-metal interaction is formed [245]. Irradiation of some systems containing Fein(Por)N3 leads to p-nitrido bridged binuclear mixed-valence complexes [(Por)Fem-N-Fe,v(Por)] [134, 162], In both cases photochemistry was used as a conventional preparative route for synthesis of the binuclear complexes. 

The decay involves attack by HO at phosphorus followed by hydride shift to cobalt or a direct hydride shift to cobalt from coordinated ammonia. The hydrido-cobalt intermediate so generated is believed to rapidly reduce another molecule of the hypophosphito complex in a post-rate-determining step to produce one equivalent of hypophosphite. 

This summary reveals the mechanistic complexity of C02-fixation processes using homogeneous catalysts. Formation of C02 insertion products into hydrido-metal intermediates could lead to metal-carboxylate or metal-formate species. At this stage it is impossible to elucidate the detailed mechanistic profiles of these transformations. In the photosystem that applies/ac-Re(bpy)(CO)3X as catalyst, O II... 

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