Exchange and Substitution Reactions

The hydroformylation of olefins is the most widely used homogeneous catalytic process using CO gas. It involves the addition of one molecule of CO and H2 to an olefin in the presence of a transition metal catalyst, most frequently based on cobalt or rhodium, resulting in the formation of an aldehyde. Generally, it is believed that the activation of H2 in cobalt-catalysed hydroformylation occurs on the unsaturated species Co2(CO)7 or Co(acylXCO)3 formed by the following reactions  

The exchange in solution of free CO with Co2(CO 8 was studied by C magnetization transfer techniques up to 7.5 MPa CO pressure [98]. The rate of CO dissociation was found to be independent of CO pressure. A slightly faster CO exchange was observed for Co(CH3COXCO)4. Ibese results suggest that the loss of CO is accompanied by the formation of an t -CH3CO-Co(CO)3 intermediate. 

Wayland and co-workers have studied the reaction of carbon monoxide with the (octaethylporphyrin)rhodium dimer, [Rh(OEP)]2 [101]. It was established by Mgh pressure and NMR spectrosopy that [Rh(OEP)]2 is in equilibrium with three new species, [Rh(OEP)]2(CO), (OEP)Rh(CO)IUi(OEP), and (OEP)Rh(CO)2Rh(OEP) [lOlJ. 

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It is possible under high pressure to photolyse Cr(CO),-(H2) further and to generate Cr(CO) ( an< then to observe exchange and substitution reactions (28). These reactions are summarised in Scheme 2 where the most significant process is the H /D2 exchange. 

Both combinations of alkyne n orbitals find filled dir orbital symmetry matches in these d4 complexes. Extended Huckel calculations on Mo(HC=CH)2(S2CNH2)2 indicate a large HOMO-LUMO gap of 1.62 eV. These octahedral complexes have proved to be quite robust and resist exchange and substitution reactions in accord with a substantial frontier orbital energy gap (153). 

One of the very characteristic features of IPECs is their ability to participate in so-called polyion interchange (exchange and substitution) reactions, previously investigated for aqueous mixtures of oppositely charged linear PEs by Kabanov... 

A general review of exchange and substitution reactions has appeared. ... 

The CO exchange and substitution reactions on these systems are usually discussed in terms of the cis and trans effects of the heteroligand X and steric factors. The rate laws are similar to that described for the M(CO)g systems given in Eq. (5.1) but, depending on the system, the or kj path may dominate. 

Some but not all of the apatite present in the body can take part in physicochemical exchange and substitution reactions. The proportion of the inorganic phase of bone which can participate in such reactions decreases as the bones become more highly mineralized. In adult animals about two-thirds of the skeletal mineral has been shown to be stable and unreactive. 

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