Group 9 rhodium and iridium

Rhodium and iridium are umeactive metals. They react with O2 or the halogens only at high temperatures (see below) and neither is attacked by aqua regia. The metals dissolve in fused alkalis. For Rh and Ir, the range of oxidation states (Table 19.3) and the stabilities of the highest mies are less than for Ru and Os. The most important states are Rh(IIl) and Ir(III), i.e. which is invariably low-spin, giving diamagnetic and kinetically inert complexes (see Section 26.2). 

When the charge is 4+ or 6+, the complexes are Ru(II)/ Ru(II) or Ru(III)/Ru(III) species respectively. For h = 5, a mixed-valence Ru(II)/Ru(III) species might be formulated but spectroscopic and structural data show the Ru centres are equivalent with charge delocalization 

Triazine acts as a bridging bgand between Ru2(02CPh)4 molecules to give a 2-dimensional coordination polymer. Surest how triazine coordinates to Ru2(02CPh)4, and predict the structure of the repeat motif that appears in one layer of the structure of the crystalline product. 

rac-c s-)Ru(bpy)2(DMSO-5)Cl can be separated into its enantiomers by HPLC using a chiral stationary phase. 

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Catalysts Prepared from Metal Carbonyls of Group 9 Cobalt, Rhodium and Iridium 331... 

Vinylidene-mediated reactions involving rhodium and iridium are discussed separately from those involving Groups 10 and 11 transition metals. The reactions of Group 9 metal vinylidenes are more numerous and have more in common with one another. Extensive stoichiometric organometallic literature aids in the understanding of these processes. In contrast, reactions of Groups 10 and 11 metal vinylidenes are more scattered and often controversial. 

When desired vinylidene-mediated pathways are not sufficiently favorable. Group 9 metal catalysts can access a set of typical side-reaction pathways. Alkyne dimerization to give conjugated enynes or higher oligomers is often observed. Polysubstituted benzenes resulting from [2 + 2 + 2] alkyne cyclotrimerization are also common coproducts. Fortunately, the selectivity of rhodium and iridium catalysts can often be modulated by the variation of spectator ligands. 

The Group 9 elements, cobalt, rhodium and iridium, have redox chemistry which in aqueous acidic solution can be summarized by Latimer diagrams ... 

Hardness of the annealed metals covers a wide range. Rhodium (up to 40%), iridium (up to 30%), and ruthenium (up to 10%) are often used to harden platinum and palladium whose intrinsic hardness and tensile strength are too low for many intended applications. Many of the properties of rhodium and iridium, Group 9 metals, are intermediate between those of Group 8 and Group 10. The mechanical and many other properties of the PGMs depend on the physical form, history, and purity of a particular metal sample. For example, electrodeposited platinum is much harder than wrought metal. 

Commercial methanol carbonylation processes have employed each of the group 9 metals, cobalt, rhodium and iridium as catalysts. In each case acid and an iodide co-catalyst are required to activate the methanol by converting it into iodomethane (CH3OH + HI CH3I + H2O) catalytic carbonylation of iodomethane into acetyl iodide is followed by hydrolysis to acetic acid. A problem common to all these processes arises because the mixture of HI and acetic acid is highly corrosive this necessitates special techniques for plant construction involving the use of expensive steels. We discuss each catalyst system in turn below. 

Complexes of the group 9 metals, especially rhodium and iridium, represent one of the more numerous families on which a systematic appraisal of struc-ture/bonding properties for the boryl ligand can be based. In part, this reflects the involvement of such systems not only in earlier work on metal-catalyzed hydroboration chemistry [2-5,35,113-123], but in more recent studies of di-boration [124,125], and the activation of C-H bonds in both saturated [9,10, 126-135] and unsaturated hydrocarbons [9,10,50,51,127,129,134,136-159]. 

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