Substitution of CO ligands

The substitutimi of a CO Ugand by another 2-electron donor (e.g. PR3) may occur by photochemical or thermal activation, either by direct reaction of the metal carbonyl and incoming ligand, or by first replacing a CO by a more labile ligand such as THF or MeCN. An example of the latter is the formation of Mo(CO)5(PPh3) (eq. 24.29) which is most effectively carried out by first making the THF (24.42) adduct in situ. 

X-ray crystallography confirms that [Rh6(P Pr3)6Hi2] possesses an octahedral Rhe-cage. Show that this cluster Fe(CO)5 - -is 10 electrons short of the expected electron count. 

In reaction 24.31, the incoming ligand provides four electrons and displaces two CO ligands. Multiple substitution by 2-electron donors is exemplified by reaction 24.32. 

In this section, we introduce the main types of ligand transformations that take place at metal centres in organometallic compounds  

Mixed carbonyl/cyanido complexes of iron were described in Section 21.9. Their importance lies in their use as biomi-metic models for [FeFe] and [NiFe]-hydrogenases (see Figs. 29.19 and 29.21). Photolysis of Fe(CO)s with [Et4N]CN leads to substitution reaction 24.33, and introduces cyanido ligands in axial positions (stmcture 24.44). 

The substitution of a CO ligand by another 2-electron donor (e.g. PR3) may occur by photochemical or thermal 

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Molecular orbital (MO) calculations on MeCo(CO)4 place a negative charge on the methyl carbon atom with polarization of the Co-Me bond. Substitution of CO ligands withdraws electron density from the alkyl carbon atom and delivers electron density to the metal, thus stabilizing the Co-C bond. 

Scheme 7 Substitution of CO ligand in carbonyl complexes by free NHC...

Most of the reactions of PH3 with the carbonyls of the transition metals and with a variety of carbonyl derivatives can essentially be classified according to three main types of reactions (i) substitution of CO ligands, of other neutral Ti-acceptor ligands like, e.g. PR3, weak 0-, S-, N-donor ligands, and alkenes, or of anionic ligands, (ii) cleavage of metal-metal bonds, and (iii) oxidative addition. Only in a few cases combinations of two or three of these reaction types or completely different reactions are observed. Most reviews dealing with the chemistry of phosphanes, with complexes of transition metals, or with transition metal carbonyls and derivatives cover only some aspects of the coordination chemistry of PH3 see e.g. [1,4, 14, 16, 22, 24, 27 to 29]. 

It was earlier observed that radical anions of binuclear metal carbonyls, generated electrochemicaUy, were unusually susceptible to attack by nucleophilic ligands. Specific substitution of CO ligands was thus achieved. This was followed by the demonstration of substitution reactions initiated by... 

In clusters Co2(CO)6(PhCCPh) and Co3(/z3-CY)(CO)9 where Y = Ph or Cl, substitution of CO ligands by PR 3 groups is initiated by reduction of the cluster. Thermal substitution is very slow. The extra electron is placed in the antibonding molecular orbital formed by the metal-metal interaction. Such reduction causes reversible metal-metal bond breaking with subsequent fast addition of the ligand to the cluster. Oxidation of the complex may occur by electron transfer either to the reagent or to the electrode ... 

Substitution of CO ligands in clusters is most commonly realized in the same way as in the case of mononuclear metal carbonyls. Substitution may be induced by one of the following, most frequently utilized methods thermal, electrochemical, chemical (reactions with N-oxide of trimethylamine or Bu"PO), photochemical, catalysis by radicals, catalysis by transition metal compounds, etc. ... 

In general, substitution of CO ligands in clusters proceeds by a dissociative mechanism as in the case of mononuclear metal carbonyls. Ligand displacement may also readily occur according to an associative mechanism in the case of complexes containing M = M double bonds as in clusters in which the M — M bond breaking takes place relatively easily (see Section 3.10a). 

As discussed in Sect. 2.4, the reaction of carbonyl metal clusters with N-oxide trimethylamine may often be a convenient method for synthesizing cluster compounds via ligand substitution. Kinetic studies of the substitution of CO-ligands by phosphines and arsines in group-18 trimetal dodecarbonyls... 

The reaction conditions necessary for the substitution of CO ligands by olefines are often not mild enough for isolating the substitution products without inducing oxidative additions processes. Thus, the reaction of Os3(CO)i2... 

Finally in this section, Bastl and co-workers have reported evidence for NaX-encaged Pt carbonyls showing reversible substitution of CO ligands by... 

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