Carbonyl hydrides, substitution

We have restricted the discussion in the section primarily to mixed carbonyl, carbonyl-hydride, or phosphine-substituted cluster compounds. On the basis of electronegativity, there should be an enhance-... 

Steric factors are important in reactions of this type. The substituted cobalt carbonyl hydrides HCo(CO)4-n(PPh3)n react with increasing difficulty as n increases (entries 32-34). Another general effect is that a hydridosilane HSiX3 will react more readily (and the product be more robust) as X becomes more electronegative (227,230). This seems to be valid for all oxidative addition processes. 

We later prepared the more stable phosphine-substituted mononuclear carbonyl hydrides such as FICo(CO)4 nLn and FHVfn(CO)5 L n= 1 and 2 ... 

L = P(C6H5)3, P(OC6H5)3 etc.] (38, 39). Interestingly, although HV(CO)6 is not known, we were able to isolate the hydride HV(CO)5P(C6H5)3 (40). In contrast to the unsubstituted carbonyl hydrides, we found that the acidity of the aqueous solutions of these compounds was considerably weakened. Recently phosphine-substituted ruthenium carbonyl hydrides, e.g., H2Ru(CO)2(PR3)2 have been reported (31). 

The coordination chemistry of iridium is concerned primarily with the I, III and IV oxidation states complexes with iridium in the — 1,0, V and VI oxidation states are also known. Of these latter states, Ir-1 and Ir° are found in carbonylate anions, oligonuclear and substituted carbonyls, hydrides and ammines. Iridium(V) and (VI) complexes are predominantly hexahalide compounds. 

Iridium(O) is present in carbonyl and substituted carbonyl complexes, and perhaps in [Ir(NH3)5], if indeed this species exists. Although the ammine complex [Ir(NH3)5] has been reported,17 the diamagnetism of this complex suggests an irridium(I) hydride species. 

The hydrides are usually obtained by acidification of carbonylate or substituted carbonylate anions. In the case of highly reduced anions like Na3Ta(CO)5, interaction with BHi" in EtOH-THF may be adequate to give [HM(CO)5]2. ... 

Known carbonyl hydrides of mthenium include the unstable HRu(CO)4, as well as the trinuclear H2Ru3(CO)n, tetranuclear H2Ru4(CO)i3 and H4Ru4(CO)i2, and complexes of even higher nuclearity, as well as substitution and deprotonation derivatives. A special feature of mthenium carbonyl chemistry is the existence of series of carbonyl... 

Thus Fe(CO)4H2 forms the salts Ca[Fc(CO)4H]a and Mg[Fe(CO)4H]j. With mercury (II) salts, however, Fe(CO)4H2 gives a precipitate of a stable, yellow, polymeric substance [Fe(CO)4Hg]n which is also made when Fe(CO)5 substituted for the carbonyl hydride ... 

Although in a few tetranuclear complexes fragmentation is the primary result of photolysis , and others, e.g., (h -CjHp Fe fCO), are inert with respect to both photoinduced M—M cleavage and carbonyl loss , substitution without fragmentation is the dominant photoreaction of tetranuclear metal carbonyls, especially of those containing second and third-row metals. Both Ir (CO),2 and H Os (CO),2 " lead to tetranuclear olefin-substituted products when irradiated with alkenes, both with some rearrangement of the Ir moiety in the former case, and a hydride abstraction in the latter. Irradiation of H Ru/CO),j in the presence of P(OMe)j or PPhj leads to stepwise formation of the substituted clusters H Ru/CO), (n = 1-4) with low quantum efficiency . Carbonyl substitution also dominates the photochemistry of HjM CCO) (M = Ru, FeRuj, FeOsj) . 

As a consequence of the recency of the development of cluster chemistry, most research in this field has been directed toward preparation of clusters and subsequent determination of their structure by X-ray studies. Sufficient preliminary studies of the reactivity of clusters, however, have been made to show that in a number of classes of reactions the closed cluster structure considerably affects the course of the reaction. The maj ority of reactivity studies have been made on the simple carbonyl clusters. A few reactions of carbonyl anion, carbonyl hydride, and substituted carbonyl clusters have also been reported. The intermediates and products of many reactions of clusters are air-sensitive and special techniques must be used to carry out reactions 347). 

Organometallic compounds with a 17-electron configuration are often labile toward associative ligand exchange. Radical chain mechanisms are well established for phosphine substitution on metal carbonyl hydrides (Scheme 23), the 17-electron chain carrier being in most cases non hydridic. This mechanism, however, was also shown to operate for OsH2(CO)4 via the 17-electron hydride complex OsH(CO)4 [137]. Thus, phosphine addition to the radical prevails over the dimerization, which indeed occurs in the absence of phosphine [33] (section 6.5.7), and over other possible decomposition pathways. The second step of the chain propagation process in Scheme 23, for this osmium system, is another example of atom transfer to a hydride radical (section 6.5.6). 

In summary of this section, it must noted that, in spite of numerous studies, nowdays we know very little about carbonyl hydrides and other substituted (mixed) carbonyls thermolysis in polymeric systems, as well as in reactive plastics. For example, in some experiments the decomposing metal carbonyls were placed into an epoxide resin heated up to the nanoparticles deposition on the forming polymer surface [121]. It is possible that the highly reactive metal particles in such systems can initiate the epoxy cycle cleavages followed by a three-dimensional space structure formation. Iron carbonyl being decomposed into polybenzimidazole suspension (in transformer oil at 473 K) forms the ferrum nanoparticles (1-11 nm) capable of polymer thermostabization [122]. 

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