Die Kristallstrukturen von LiCo(CO)4und NaCo(CO)4

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Abstract
The crystal structures of solvent-free crystals of two ionic tetracarbonylcobaltates are described. In LiCo(CO) 4 ( P [unk]3 m , a = 554.2 pm, Z = 1) [Co(CO) 4 ] − -ions are regular tetrahedral by space group symmetry. The structure is discussed as a cubic dense CO-packing with Li- and Co-atoms in tetrahedral voids. The structure of NaCo(CO) 4 ( Pbcm , a = 552.8, b = 1374.1, c = 920.9 pm, Z = 4) is built by distorted tetrahedral [Co(CO) 4 ] − -and Na + -ions. The Na-atoms are coordinated by 4 O-atoms forming a distorted square, and by the Co(CO) 3 -moiety of a Co(CO) 4- tetrahedron ( d Na–co = 318.2 pm). This structure can be considered as a distorted cubic dense CO-packing with interstitial Na- and Co-atoms, intermediate between the LiCo(CO) 4- and the KCo(CO) 4- structure. The shape of the [Co(CO) 4 ] − -ion could be derived from structural data of 5 alkali-tetracarbonylcobaltates by subtracting the ionic radii of A from A –CO-lengths. A – CO-contacts are of two types: “Polar” contacts (p) C – O ··· A and “normal” contacts (n) C – O, which are ≈30pm longer. [unk] Coordination numbers of the alkali ions in A Co(CO) 4 -compounds derived from the van der Waals-shape of the anion increase from Li to Cs via an increasing number of normal contacts (Li: 4p + 0n, Na: 4p + 3n, K: 4p + 5n, Rb: 4p + 6n, Cs: 4p + 8n).

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