Studies of transition metal nitrite complexes

A series of transition metal nitrite complexes have been prepared. These include tetranitrite complex ions of the formula [M(NO\\(_2\\))\\(_4\\)]\\(^{2-}\\), M = Zn(II), Cd(II) and Hg(II), a series of nickel(TT) nitrite complexes with amines of varying degrees of substitution and the pentammine nitrite complexes [Co(NH\\(_3\\))\\(_5\\)NO\\(_2\\)]\\(^{2+}\\), [Co(NH\\(_3\\))\\(_5\\)ONO]\\(^{2+}\\) and [Cr(NH\\(_3\\))\\(_5\\)ONO]\\(^{2+}\\). Single crystal optical spectra were measured for the Co(III), Cr(III) and Ni(II) complexes. The observed transition energies for these complexes are compared with those calculated using the Angular Overlap Model (AOM). Bonding parameters have been derived for the nitrite ligands, and the parameters are shown to be transferable to other complexes with the same transition metal ion and different nitrite coordination modes. An extended version of the AOM developed to include the effects of\bent bonding\" was used to derive bonding parameters for OO' chelating and NOO' -bridging nitrite ligands. The bonding parameter e\\(_˜ìvâ\\) deduced for N-bonded and O-bonded nitrites implies that the N-bonded ligand is a marginally stronger a-donor than the O-bonded ligand. The e\\(_{˜ìvÑy}\\) parameters derived for the nitrito and nitro coordination modes suggest that the O-bonded nitrite is a moderate ˜ìvÑ-donor and the N-bonded nitrite a weak ˜ìvÑ-acceptor. The large difference in the overall field strength and very different position in the spectrochemical series is due mainly to the difference in the ˜ìvÑ-bonding of the two coordination modes. The complexes [Ni(NH\\(_3\\))\\(_4\\)(NO\\(_2\\))\\(_2\\)] [Ni(en)\\(_2\\)(NO\\(_2\\))\\(_2\\)] and [Ni(NN-dimen)\\(_2\\)(ONO)\\(_2\\)] (NN-dimen = NN-dimethylethylenediamine) have been modelled using Density Functional Theory (DFT) calculations. The calculated energies for the nitro and nitrito isomers of the above complexes suggest that the nitro isomer is inherently slightly more stable than the nitrito isomer. This agreed with observation in the solid state for [Ni(NH\\(_3\\))\\(_4\\)(NO\\(_2\\))\\(_2\\)] and [Ni(en)\\(_2\\)(NO\\(_2\\))\\(_2\\)] but not for the complex [Ni(NN-dimen)\\(_2\\)(ONO)\\(_2\\)] though in chloroform solution the nitro isomer is slightly more stable for the last complex. In agreement with this trend the difference in the energies calculated for both isomers was the smallest for the complex with the substituted amines. The calculations confirm that the nitrite acts as ˜ìvÑ-donor when O-bonded compared with the N-bonded form and ˜ívÆ values were derived that are comparable to those derived via the AOM and estimated from the optical spectra. Infrared and Raman spectra were measured for each complex prepared in the present work to unambiguously assign the fundamental nitrite vibrations (v\\(_1\\) v\\(_2\\) and v\\(_3\\)). Upon nitrito coordination v\\(_2\\) shifts significantly to lower energy compared with the ionic stretch. For nitro coordination this vibration shifts to higher energy decreasing the energy difference between v\\(_2\\) and v\\(_3\\). The energies of the nitrite stretching vibrations shift to slightly lower energy than those of the free ion upon OO' -chelating coordination. The cis-NO-bridging mode did not have a great effect on the vibrational energies of the nitrite wtlike the trans NO-bridging mode where v\\(_2\\) shifts to lower energy and V\\(_3\\) shifts to much higher energy relative to free nitrite."