MOLECULAR-ORGANIZATION IN CRYSTALLINE [CO2(CO)8] AND [FE2(CO)9] AND A SEARCH FOR ALTERNATIVE PACKINGS FOR [CO2(CO)8]

The molecular organization in crystals of [Co2(CO)8] and [Fe2(CO)9] has been investigated by means of packing potential-energy calculations and computer graphic analysis. It is shown that the two molecules pack in nearly identical ways in their crystals. However, while crystalline [Fe2(CO)9] is compact, solid [Co2(CO)8] presents large empty channels (van der Waals width between 2.4 and 4.5 angstrom, at room temperature) corresponding to the approximate location of possible residual electron density (whether due to Co-Co bonding electrons or to lone pairs on the Co atoms). Alternative crystal packings for [Co2(CO)8] have been generated, using improved atomic potential parameters adapted to carbonyl compounds. On purely steric grounds, it is shown that the [Co2(CO)8] molecule is compatible with more close-packed crystal arrangements than the one actually observed. In two alternative triclinic and monoclinic crystal structures the 'empty' bridging sites are shown to be available for molecular interlocking.