Six-coordinate carbon: 9,9-Dilithiofluorene–thf complex (see picture) crystallizes as chains of lithium atoms which are alternately bridged by fluorenylidene and THF units. These polymer chains are held together by Li–C π and σ interactions; the tetrametallated carbon atom is six-coordinated. This means, structural elements are found here, which are known from lithioalkanes, on the one hand, and from the lithocene anion, on the other hand.
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Linti et al. (2002) studied this question.