Carbonyls, metal photochemical substitution

Photochemical substitutions on metal carbonyls and their derivatives. W. Strohmeier, Angew. Chem., Int. Ed. Engl., 1964, 3, 730-737 (68). 

A series of stable organometallic SO2 complexes of Cr, W, and Mn, including CpMn(C0)2(S02), has been synthesized by photochemical substitution of carbonyl ligands. Unstable pentacarbonyls, M(C0)5(S02) (M = Cr, W), were claimed but not isolated The complex CpMn(C0)2(S02) was found to contain j -planar SO2 (M-S = 2.037(5) A) and is one of the few sublimable metal-S02 complexes The SO2 ligand lies approximately in the plane which also contains the Mn atom and one atom of the Cp ring (see Fig. 5). The bonding has been discussed by Hofhnaim and coworkers who concluded that the observed orientation allows interaction between the best 71 acceptor orbital of SO2 and the best jt donor orbital of the CpMn(CO)2 fragment ... 

Chromium hexacarbonyl is extremely photolabile (equation 6) therefore photochemical substitution is an efficient means of preparing derivatives. Oxidation of the Cr center requires nitric or sulfuric acid, or chlorine. Alternatively, some hgands induce complete carbonyl dissociation with concomitant oxidation, for example, acetylacetonate. Chemical reduction with alkali or alkaline-earth metals or electrochemical reduction proceeds in two-electron steps with loss of two CO molecules to first give [Cr2(CO)io]" and then [Cr(CO)s]. Nucleophilic attack at CO generates a number of stable (Nu = R) and unstable (Nu = N3, OH, H, NEt2) products. The stable [(OC)5CrCOR] ion is a carbene precursor. 

Nitrile derivatives of the metal carbonyls have been discussed together with other nitrogen donor molecules in a number of contexts. Much of the early work has been reviewed by Manuel 337) in his article on Lewis base-metal carbonyl complexes in Volume 3 of this series, and by Stroh-meier 436) in his review of photochemical substitution reactions. In general, nitriles are weaker Lewis donors than phosphorus and nitrogen bases 436), but compared to carbon monoxide, better electron donors but poorer acceptors 427). Force constants and assignments for a series of complexes [(MeCN)jjM(CO)g J (M = Cr, Mo, W) were studied 165, 228, 296) and... 

Photochemical substitutions on 8 metal carbonyls and their deriva- (68) tives... 

Similar photoinduced dimerizations and ligand substitutions in the presence of additives such as triphenylphosphine are observed with ion-pairs salts of Mn(CO)s and V(CO)6" with cobaltocenium or other cationic acceptors such as Ph2Cr", pyr-idinium, quinolinium, etc [118]. Most importantly, all photochemical transformations of the various carbonyl metallate salts are initiated by actinic light that solely excites the charge-transfer absorption bands of the contact ion pairs whereas local excitation of the separate ions is deliberately excluded. 

There are several recent reviews of the photochemistry of transition metal complexes 3,4,5,29,30,45,466,514,518,580)> the most comprehensive being 4>. However, these papers deal mainly with ionic coordination compounds containing inorganic ligands only one 466> is devoted to photochemical substitution reactions of metal carbonyls and their derivatives, while in another 4> this subject is discussed in a short paragraph. 

The photochemical substitution of metal carbonyls has found wide application in the preparation of a fascinating variety of transition metal w-donor complexes. In addition to the complexes listed in Table 4 a few examples are shown in more detail to illustrate the preparative possibilities. 

Photochemistry of Metal Carbonyls, Metallocenes, and Olefin Complexes Table 4. Photochemical substitution of metal carbonyl compounds with n-donors... 

Table 5. Photochemical substitution of metal carbonyl compounds by n-donors 1. Monoolefins M(CO) iL, M(CO)B-2L2...

Table 5 Photochemical substitution reactions of metal carbonyl compounds Substrate... 

An infrared spectroscopic study of the photochemical substitution and oxidative addition reactions of Cp M(CO)4 compounds of the Group 5 Metals has been carried out and, lastly, Delmata et have reported phosphine and carbonyl... 

Reactions involving thermal or photochemical substitution of coordinated carbon monoxide in metal carbonyl complexes are well known (see Chapter 3) and related processes may be induced electrochemically. A number of review articles blS have dealt with the electrochemically-induced... 

Selective Photochemistry. It is well known that for many substituted metal carbonyl compounds in solution, photochemical loss of either CO or the substituent L can be promoted depending... 

The dominant photochemical reaction of metal carbonyl compounds is loss of carbon monoxide, which is usually followed by substitution of another ligand to replace the expelled carbon monoxide. 

Phosphorus trifluoride is a ligand that is used extensively in coordination chemistry. It substitutes readily into various metal carbonyl complexes using either thermal or photochemical techniques. As a ligand, it is unique in its similarity to carbon monoxide in lower-valent organometallic compounds. In its role as a model for CO, a number of studies are possible that cannot be done on the carbonyls themselves.1 The name normally used for PF3 in complexes is trifluorophosphine. 

The most straightforward route to heteroaldehyde and heteroketone complexes is the substitution of a heterocarbonyl compound for another coordinated ligand. This method is naturally restricted to heteroaldehydes and heteroketones stable in the uncoordinated form, i.e., is usually restricted to thioketones and a few stable seleno- and tellurocarbonyl compounds.141718,27118 In most cases, metal carbonyls or solvent complexes of metal carbonyls were used as the complex precursors. The photochemically or thermally induced loss of the ligand to be replaced is followed by coordination of the heterocarbonyl compound [Eq. (3)]. 

The range of alkenes that may be used as substrates in these reactions is vast Suitable catalysts may be chosen to permit use of ordinary alkenes, electron deficient alkenes such as a,(3-unsaturated carbonyl compounds, and very electron rich alkenes such as enol ethers. These reactions are generally stereospecific, and they often exhibit syn stereoselectivity, as was also mentioned for the photochemical reactions earlier. Several optically active catalysts and several types of chiral auxiliaries contained in either the al-kene substrates or the diazo compounds have been studied in asymmetric cyclopropanation reactions, but diazocarbonyl compounds, rather than simple diazoalkanes, have been used in most of these studies. When more than one possible site of cyclopropanation exists, reactions of less highly substituted alkenes are often seen, whereas the photochemical reactions often occur predominantly at more highly substituted double bonds. However, the regioselectivity of the metal-catalyzed reactions can be very dependent upon the particular catalyst chosen for the reaction. 

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