Substitution of carbonyl ligands

Oxidative addition to complex 1 is the slowest and rate-determining step in the reaction scheme and also it is a singular step, involving the conversion of the catalyst resting state to a more reactive 2. An obvious way to obtain a faster catalyst is the substitution of carbonyl ligands in 1 by electron-donating phosphines, as organometallic chemistry tells us this variation never fails. Indeed, several variants that are indeed fester are known [11], but none of them has found application. 

Rhenium(0) compounds are rare and frequently lie in the realm of the organometallic chemistry. A simple example is decacarbonyldirhenium(0) in which two staggered, square-pyramidal Re(CO)5 fragments are held together by a single rhenium-rhenium bond. Substitution of carbonyl ligands is possible by tertiary phosphines and arsines, silanes and isocyanides, and binuclear Re-Re, Mn-Re, and Co-Re complexes have been studied. " Successive replacement of CO ligands can readily be observed by vibrational spectroscopy. This has been demonstrated... 

Photochemical activation of transition metal carbonyls has been used as a preparative tool for substitution of carbonyl ligands by donor molecules or unsaturated hydrocarbons for many years (7-6). The advantage of photochemical activation in comparison with thermal activation is the possibility of conducting reactions at fairly low temperatures. Hence even thermolabile products can be prepared and isolated by appropriate treatment of the reaction mixtures. However, due to the various activation modes of transition metal carbonyls by UV light, often more than one product is obtained, and chromatographic separation is necessary. Limitations are set primarily by the amount of substance which can be irradiated in solution at one time. 

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 ... 

A number of derivatives of this complex have been prepared (see Multi-Heme Cytochromes Enzymes). Substitution of carbonyl ligands by other donors, such as that shown in equation (15), gives rise to a wide range of complexes. 

The substitution of carbonyl ligands in W(CO)6 may be effected either thermally or photochemically. Among the more important reactions from a synthetic standpoint are those of W(CO)6 with nitriles (RCN) to generate yoc-W(C0)3(RCN)3 (3) (equation 1), the nitrile ligands of which may be displaced by other Lewis bases see Lewis Acids Bases) under mild conditions. The reactions of W(CO)6 with the cyclopentadienyl see Cyclo-pentadienyl) reagents MCp (M = Li, Na, K Cp = Cp,... 

The reduction of [Fe3 ( 3-S)2 (CO)g (tSCHCHli) ] in DMF is reversible at fast cyclic voltammetric sweep rates ( = -0.73 V). In the presence of P(OMe)3, three new reduction waves are observed, revealing the stepwise substitution of carbonyl ligands to give [Fe3(/X3-S)2(CO)8 P(OMe)3 (< SCHCHS)] n = 1-3). Under 1 atm carbon monoxide, the rate of substitution was less, leading thd authors to favor a mechanism involving CO displacement after the initial reduction step, rather than rupture of the M—M bond before ligand loss. In either case, reduction clearly activates the cluster toward carbonyl substitution 150). 

Scheme 10. The stepwise substitution of carbonyl ligands by RNC (R = f-Bu or p-tolyl) in the derivative [Rh2(/i-pz)(/j-dppm)2I2(CO)2l1 is accompanied by the intermediate formation of species containing a bridged carbonyl ligand (see text). [Adapted from (161).]...

The substitution of carbonyl ligands in Vcp(CO)4 is catalyzed by PdO. Products of these reactions are complexes of the type [Vcp(CNR) (CO)4 ]. The substitution of CO groups may also be induced electrochemically. 

Special Ligands. As discussed in Sect 2.5.3, the substitution of carbonyl ligands in the cluster often enhances the liability of the clusters. This property has also been used in catalytic processes. Thus for instance, the nickel cluster with isocyanide ligands Ni4(CNR)7 is an efficient catalyst precursor in the hydrogenation of acetylenes. Actually, Ni4(CNR)7 reacts with acetylenes yielding the adduct Ni4(CNR)4 (acetylene)3 which in turn is able to catalyze, under rather mild conditions, the hydrogenation of diaryl and dialkylacetylenes at rates of ca. one turnover per minute. Mononuclear complexes do not catalyze this reaction under the same conditions. 

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