hel late.^'^ Lie is not bound to a C atom of the fluorenyl anion, the shortest distance Li-C13 is 403.6 pm (in the solid state structure of the contact ion pair fluorenyllithium ' 2 q u i n ~c l i d i n e " ~~ the following Li-C distances were found: 233.3, 247.1 and 279.4 pm). In contrast, the fluorenyl anion 1 exhibits hydrogen bonding with the amine hydrogen atoms of the EDAs, of which only those with H . . . C distances < 280 pm are shown in Figure 1. The bonds H14-Cl (250 pm), HlO-Cl (257 pm) and H17-C7 (259 pm), in particular, are significantly shorter than the sum of the van-der-Waals radii of H and C (290 pm).["l According to semiempirical ' ' I C1 exhibits the highest charge density, that of C7 also being high. Up to now the presence of H bonds to carbanions was mainly concluded from kinetic investigations ;[l3I spectroscopic detection was possible in only a few cases.['41 Investigations by Seebach et al.I''] and Tamm et a1.[I7l show the significance of N-H bridged amide, ester, and ketone enolates, which were prepared by deprotonation with lithium amides, in alkylation or deuteration (degree of deuteration up to <1%). Dunitz and Seebach et al. first found an N-H . . . C bond X-ray crystallographically in the lithium enolate [tBuC(OLi)CH,. TriMEDA]2.1'5b1 Our findings are in agreement with the UV/VIS spectrum of fluorenyllithium in ethylenediamine, from which the presence of N-H.. . C hydrogen bonds had already been concluded;['*] the absorption maximum A,,, = 368 nm"91
is . shifted hypsochromically compared to that of the solvent-separated ion pair (FIQ, THF, Lie, -3O"C, A,,,= 373),"01 and the contact-ion pair (Fl', Li', THF, 25°C) absorbs at 349 nm. [19] Marcus["1 recently made a distinction between "solvent separated", "contact" and "solvent shared ion pairs. In a solvent-separated ion pair each ion has its own solvent shell, whereas in a solvent-shared (solvent-bridged) ion pair both ions share the solvent molecules. The solid state structure of 1 is thus a model for a solvent-bridged ion pair. In this case in particular it is also to be expected that the N-H bonds participating in H-bonds are additionally polarized by the Li'-N interaction and thus better hydrogen bond donors. Solvent-shared ion pairs of the type described here should therefore be of general importance in polar organometallic compounds in solvents which contain N-H or 0 -H groups.