Bis[bis(trimethylsilyl)amido]iron(II), Fe[N(SiMe3)2]2, has been prepared from FeBr2(THF)2 and LiN(SiMe3)2. The corresponding Co(II) and Mn(II) amides are known to be dimeric in the crystalline phase, and the Co amide is known to be monomeric in freezing benzene. All three amides are monomeric in the gas phase at 130-150 °C/1 Torr. Gas electron diffraction data are consistent with monomers of S4 symmetry (which implies zNMN = 180°) and bond distances of Mn-N = 195 (2), Fe-N = 184 (2), and Co-N = 184 (2) pm. Intraligand strain is reduced by opening of the SiNSi angle to 130° and rotation of SiMe3 groups. Comparison with the structures of the amido-bridged dimers of the Mn and Co amides shows that dimer formation is accompanied by significant elongation of the terminal M-N bonds and compression of the SiNSi valence angle of the terminal ligand by about 10°. Both changes are interpreted as evidence for ligand-ligand repulsion in the dimers. SCF MO calculations on high-spin 6A[Mn(NH2)2 yield an equilibrium bond distance of Mn-N = 193 pm. It is concluded that the monomeric Mn amide is high spin. The SCF calculations suggest that the Mn-N bonding is very polar and that N-Mn pπ-dπ bonding is negligible. The photoelectron (PE) spectra of the Mn and Fe amides have been recorded and assigned. The similarity of the PE spectra of the Fe and Co amides to that of the Mn analogue suggests (but does not prove) that these species are high-spin 2B1 and 3A2, respectively. © 1988, American Chemical Society. All rights reserved.
