Iridium dihydride, bonding

The use of CO2 as a reagent for synthetic purposes would be highly desirable, due not only to the vast availabiUty of this gas but also its environmental concerns. The stoichiometric activation of CO2 has been achieved with the iridium-PCP complex 29 comprising an alkyl rather than an aryl skeleton (Scheme 12.12) [32]. The addition of CO2 to the dihydride complex results in C=0 insertion into the iridium-hydride bond, and affords the formate complex 30. However, this complex is not stable and disproportionates spontaneously into the virtually insoluble bicarbonate complex 31 and the carbonyl dihydride 32. Such disproportionation is suppressed when the iridium metal center is replaced by rhodium [33], which is generally assumed to have a lower hydride affinity than iridium. 

MLX plots and electron count, 1, 36 MLX plots and ligand bond number, 1, 42 MLX plots and reactivity patterns, 1, 42 MLX plots and valence, 1, 40 multicenter bonding, 1, 32 pi- and delta-backbonding, 1, 29 pi-donation considerations, 1, 29 Covalent character, dn configuration compounds, 1, 20 Covalent interaction studies, and bis(arene)chromium, 5, 345 CPPL, see Circularly polarized photoluminescence Crabtree s catalyst, for iridium dihydride complexes,... 

In the Graham system, irradiation of the carbonyl iridium complex 1 extrudes a CO molecule and leads presumably to the coordinatively unsaturated intermediate 2 which then inserts into the CH bonds of cyclohexane or neopentane giving the compounds 3. Similarly, Bergman found that the iridium dihydride 4 loses H2 upon irradiation when the reaction takes place in cyclohexane or neopentane, the hydrido alkyl compounds 6 are formed, resulting from the oxidative addition of a CH bond to the intermediate 5. 

The reduction behaviour of the alkylidene adduct of a cobalt-dithiolene complex (423) has been examined548 and the study has shown that, when the alkylidene-bridged structure (423) is reduced by one electron, it isomerizes rapidly and quantitatively to the ylide form (424). This represents the first example of reversible isomerization of the metal-carbon bond in a cobaltadithiolene complex. A surprising cis- to tra .s-dihydride isomerization which is unprecedented for 18-electron six-coordinate complexes has been observed549 in an octahedral iridium-c7.y-di hydride complex. 

Reviews during late 1988 and 1989 relevant to metal-hydrides include a treatment of H2 oxidative addition and dihydride transfer reactions involving [Ir(X)(CO)L2]. Stereochemical aspects are emphasized. A discussion of polynuclear iridium hydrides has also appeared. The relative bond strengths of M—H... 

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