Bond energies Borane carbonyls

Figure 3.27 shows reaction equations and the energy relationships of the hydroboration of enantiomerically pure a-pinene with 9-BBN. The reagent approaches only the side of the C=C double bond that lies opposite the isopropylidene bridge. The addition is thus completely diastereoselective. Moreover, the trialkylborane obtained is a pure enantiomer, since the starting material is a pure enantiomer. It is used as Alpine-Borane for the enantioselec-tive reduction of carbonyl compounds (Section 10.4). 

The occurrence of stable neutral. binary carbonyls is restricted to the central area of the d block (Table 19.3), where there are low-lying vacant metal orbitals to accept o-donated lone-pairs and also filled d orbitals for jr back donation. Outside this area carbonyls are either very unstable (e.g, Cu, Ag, p. 1199), or anionic, or require additional ligands besides CO for stabilization. As with boranes and carboranes (p, 181), CO can be replaced by isoelectronic equivalents such as 2e , H , 2H or L. Mean bond dissociation energies Z>(M-CO)/kJ mol increase in the sequence Cr(CO)fi 109, MoCCO>6 151. W(CO)6 176, and in the sequence Mn2(CO)so lOO, Fe(CO)s 121, Co2(CO)g 138. NiCCO)4 147. 

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