Molecular organization in crystalline [Co2(CO)8] and [Fe2(CO)9] and a search for alternative packings for [Co2(CO)8]

Abstract

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 Å, 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.