The disadvantages of the system as already mentioned

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Unformatted text preview: rmediate to which the alkene can attach itself. The disadvantages of the system, as already mentioned, are its lack of specificity, leading to branched-chain products, and the necessity of high temperatures (> 150°C) and pressure (-200atm). In addition the volatility of [HCo(CO)4] poses recovery problems. The dinuclear octacarbonyls are obtained by heating the metal (or in the case of iridium, IrC13 + copper metal) under a high pressure of CO (200-300atm). Co2(CO)8 is by far the best known, the other two being poorly characterized; i t is an air-sensitive, orange-red solid melting at 38 J . E. ELLIS, dv. Organometallic Chem., 31, 1-52 (1990). A Ch. 26 5 1°C. The structure, which involves two bridging carbonyl groups as shown in Fig. 26.8a, can perhaps be most easily rationalized on the basis of a “bent” Co-Co bond arising from overlap of angled metal orbitals (d2sp3 hybrids). However, in solution this structure is in equilibrium with a second form (Fig. 26.8b) which has no bridging carbonyls and is held together solely by a Co Co bond. The most stable carbonyls of rhodium and iridium are respectively red and yellow solids of the form [M4(C0)12] which are obtained by heating MC13 with copper metal under about 200 atm of CO. The black cobalt analogue is more simply obtained by heating [co,(c0)8] in an inert atmosphere - 50°C + ~ [ C O ~ ( C O ) ~ I [ C O ~ ( C O ) ~4 I ~ 0 ~ The structures are shown in Fig. 2 6 . 8 ~ d and and differ in that, whereas the Ir compound consists of a tetrahedron of metal atoms held together solely by M-M bonds, the Rh and Co compounds each incorporate 3 bridging carbonyls. A similar difference was noted in t...
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