The molecule containing Fe2 shows a hydrogen bond between one NH3 proton and one uncoordinated THF. Hence, the shorter Fe-N length in Fe2 may be due to the hydrogen bond, which increases the basicity of the NH3. While the Fe-S bond lengths of 10 are similar to those in 8-crown and 9-crown, the Fe-C bond length is significantly shorter in 10 (Table ). This difference may arise because the thiolate and amido ligands exert a larger trans influence than the ammine. The solid-state IR spectrum of 10 shows four weak bands at 3352, 3299, 3236, and 3159 cm -1 that are assigned as N-H stretches (Figure ). Their frequencies are not suggestive of any significant coordination-induced N-H bond weakening. The 1 H NMR spectrum of 10 in THF-d8 has nine resonances between 175 ppm and -78 ppm, consistent with C2v symmetry (Figure ). Since the complex is Cs symmetric with THF coordination on one face, the spectrum in THF-d8 could indicate fast THF exchange on both sides of the complex. Its spectrum in C6D6 has severe broadening compared to THF-d8, and there are at least 15 resonances (Figure ). Assuming that coordinated THF has two different proton environments and that NH3 rotation on the NMR timescale renders its protons equivalent, a Cs symmetric geometry (with the mirror plane perpendicular to the pincer) would predict 14 peaks. Thus, the number of peaks in the C6D6solvated NMR spectrum suggests 10 has no symmetry in C6D6, though the reason for the low symmetry is not obvious. ). Low isomer shifts such as that of 11 are frequently observed in iron carbonyl complexes, reflecting the withdrawal of d-electron density from the iron nucleus by backbonding. The 1 H and C NMR spectra of 11 show 7 and 14 narrow peaks, respectively (Figures and). These spectra are consistent with a C2v-symmetric diamagnetic species.