Photosubstitution of CO

Let us consider [W(CO) ] as the prototypical example. The rate constant for CO dissociation in this compound is I0 s at STP. Tungsten(0) is LS and has the electron configuration which makes it thermally inert. Irradiation of the 

Some examples of photosubstitution reactions of metal carbonyls. 

Photoexcitation of the T,g Aig transition in [W(CO)g] is followed by intersystem crossing to the reactive T,g ES, which dissociates a CO ligand. The W-C bond is weakened in two ways by removal of an electron from the t2g level and by population of the antibonding Og MO. 

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Preparative reactions involving photosubstitution of CO are carried out by irradiation of the compound in a weakly-coordinating solvent such as tetrahydrofuran (THF) and then displacement of the solvent ligand at room temperature with the chosen ligand, L. 

Photosubstitution of CO in Fe(CO)3(Me2N4) and Fe(CO)2L(Me2N4) complexes proceeds via a dissociative mechanism and allows the synthesis of a variety of mixed ligand iron tetraazadiene complexes (see Table 5).177... 

On the basis of these results we propose the mechanism shown in Figure 9 for the photolysis of these complexes at T <200K. This reaction is observed for all complexes (CO)sMMn(CO)3(a-diimine). Irradiation into the MLCT band causes photosubstitution of CO of the Mn(CO)3(a-diimine) moiety by 2-Me-THF. The difference in electronegativity between both metal fragments is then so large that raising the temperature causes a heterolytic splitting of the metal-metal bond. The cation formed reacts with CO from the solution. When the temperature is raised to room temperature the ions recombine to the parent compound because 2-Me-THF is then released. 

Figure 3- Electronic absorption spectrum of Fe(C0)T(l, 4-4102%) and quantum yields for photosubstitution of CO by PPI13. Reproduced from Ref. 21. Copyright 1981, American Chemical Society.

Table 11. Photosubstitution of Co(III)-amines upon 488nm photolysis at 25°1)...

An atom transfer chain mechanism very similar to that in Scheme 6 is thought to occur in some photoinitiated CO substitution reactions of organometallic halide complexes. For example, [Cp2Mo2(CO)6] promotes the photosubstitution of CO by PR3 in [CpMo(CO)3X] (X = Br, I),"9 and... 

Cp2Fe2(CO)4] promotes photosubstitution of CO by RNC in [CpFe (CO)2I].120 The propagation step in these reactions is halogen atom abstraction from reactant by the 17-electron radicals [CpMo(CO)2PR3] and [CpFe (CO)2(CNR)], respectively. A radical chain pathway has also been proposed for the [Cp2Fe2(CO)4]-promoted photosubstitution in Eq. (27).121 In... 

These reactions may be identified by the dependence of the quantum yield for the reaction on the concentration of entering ligand. An exemplary case, in which the photosubstitution of CO occurs partially by the associative mechanism outlined above, was studied by Keeton and Basolo (59) ... 

An example of case 1.1 is provided by the photosubstitution of CO ligands by L (L = PBu3, PPh3) in HRe(CO)s in the presence of catalytic quantities of Re2(CO)io or Mn2(CO)2, for which a possible mechanism is shown in Scheme 2. This mechanism is not the one proposed in the original report [17] it illustrates the intervention of uneven catalytic cycles (contrasted with the ones involving species... 

Photolysis of the bimetallic complex (29) in argon matrices resulted in CO loss only from the Mn end of the molecule, yielding [(CO)3Mn(p,-Ti T] -C3H4C6Hs)Cr(CO)3] as the sole observed product. In a nitrogen matrix, on the other hand, photosubstitution of CO with N2 was observed at both metal centres. The reactive intermediate (i-Pr2NP)2Fe2(CO)6 has been generated from... 

TABLE 7 Quantum Yields and Volumes of Activation for the Photosubstitution of CO in M(CO)4phen by PEt3 in Toluene"... 

Because of r-backbonding, metal carbonyls are usually quite stable in their ground states. Irradiation of metal carbonyls, on the other hand, often leads to photosubstitution of CO. Several examples are shown in Figure 19.34. This process can be quite useful from a synthetic point of view when replacement of a single CO ligand is desired. 

The quantum efficiency of photosubstitution of CO in [M(C0)5L] compounds decreases when L is a good acceptor ligand because the MLCT excited state is now longer in energy than the LF excited state. 

M—M Bonds Another important photochemical process is the homolysis of M—M bonds. The fragments produced are likely to be odd-electron and therefore substitutionally labile. For example, the photosubstitution of CO in Mn2COio by PPh3 proceeds via the 17e intermediates Mn(CO)5. Equation 4.57 is an interesting example, because the replacement of three COs by the non-ir-acceptor NH3 leads to a buildup of electron density on the metal. This is relieved by an electron transfer from a 19e Mn(CO)3(NH3)3 intermediate to a 17e Mn(CO)5 fragment to give the disproportionation product 4.20 in a chain mechanism. ... 

Photosubstitution of CO shown in equation (7) occurs with phosphines, phosphites, and CO and has a quantum yield independent of the nature or concentration of Rate-determining CO dissociation from the photoexcited reactant is indicated. Similar chemistry obtains with [CpRu(CO)2Me], except that irradiation at 77 K allowed observation of the [CpRu(CO)Me] intermediate. This was not possible with the iron analog. 

The substitution of E2Ph2 in (14) (E == Se or Te) by CO follows a dissociative pathway with Se > Te in rate/" Photosubstitution of CO by PPh3 and P(0-i-Pr)3 in [Cp2pe2(CO)4] does not occur via [CpFe(CO)2] radicals as might be expected. Instead, (15) is postulated to be the reactive intermediate formed by photolysis that actually leads to observed products. " ... 

In Section 11.2.B, we showed that the concerted or least-motion dimerization of two olefins requires excessively high activation energies. This is the classic case of a symmetry-forbidden reaction. A two-step reaction mechanism, or at least a different reaction path, has to be followed. In this section, a somewhat analogous reaction, the dimerization of two olefins in the presence of Fe(CO)s and CO [21 ], is investigated. An initial sequence In this reaction is the photosubstitution of CO by two olefins on Fe(CO)s which gives the 18-electron intermediate, 18.26. It rearranges to the 16-electron metallacyclopentane, 18.27, wherein one C—C and two Fe—C bonds... 

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