Substitution and Exchange Reactions of CO Ligands

Related to catalytic methanol carbonylation reactions, substitution of iodide by CO in [M(CO)2l2r (M = Rh, Ir) has been studied by HP IR spectroscopy [103]. Under CO pressure, in CHCI3, both complexes were found to generate the corresponding neutral tricarbonyl, [M(CO)3l]. The Rh species had already been reported by Morris and Tinker in the reaction of [Rh(CO)2l]2 with CO [104] and the Ir species is observed under certain conditions during Ir-catalysed methanol carbonylation [39, 60]. While, for M = Ir, 35% conversion was attained at 150 bar CO, for Rh less than 4 % conversion occurred. 

Equilibrium constants for Eq. (8) were determined as = 1.8 X 10 and Kri, = 4 X 10 from the HP IR spectroscopic data, indicating a higher affinity for CO (relative to I ) for Ir(I) than Rh(I). The same study also reported the HP IR observation of two isomers of [H2lr(CO)2l2], from the oxidative addition of H2 to [Ir(CO)2l2r. In combination with HP NMR data, it was found that the initially 

Cationic Group 11 carbonyl complexes have been implicated in catalytic carbony-lation reactions of alkenes, arenes, alcohols, saturated hydrocarbons and aldehydes under acidic conditions [106]. While the mono- and di-carbonyls [M(CO)] and [M(C0)2] (M = Cu, Ag, Au) can be formed at atmospheric pressure of CO, only Cu(I) forms a tricarbonyl cation under such conditions [107]. Strauss and co-workers reported the observation of [Ag(CO)3] when a Fluorolube mull of Ag[Nb(OTep5)5] was subjected to 13 bar CO in an HP IR transmission cell [108]. 

The gold analog, [AujCOjj] is reported to be formed (with an [Sb2Fu] counterion) only under 100 bar CO [107]. The y(CO) bands of these species occur significantly above the stretching frequency of free CO (2143 cm ) for example the silver and gold tricarbonyl cations have IR absorptions at 2192 and 2212 cm respectively. This is taken to indicate that there is little or no metal — CO Ti-backdonation in these species, which are termed non-classical metal carbonyls [109]. 

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