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Singlet-Triplet Gaps of Cobalt Nitrosyls: Insights from Tropocoronand Complexes
Kathrin H Hopmann1, Jeanet Conradie1,2, Espen Tangen1
1†Department of Chemistry and Center for Theoretical and Computational Chemistry, University of Tromsø, N-9037 Tromsø, Norway.
Abstract:
A density functional theory (DFT) study of {CoNO}(8) cobalt nitrosyl complexes containing the [n,n]tropocoronand ligand (TC-n,n) has revealed a sharp reduction of singlet-triplet gaps as the structures change from near-square-pyramidal (for n = 3) to trigonal-bipyramidal with an equatorial NO (for n = 5, 6). An experimental reinvestigation of [Co(TC-3,3)(NO)] has confirmed that it is not paramagnetic, as originally reported, but diamagnetic, like all other {CoNO}(8) complexes. Furthermore, DFT calculations indicate a substantial singlet-triplet gap of about half an eV or higher for this complex. At the other end of the series, low-energy, thermally accessible triplet states are predicted for [Co(TC-6,6)(NO)]. Enhanced triplet-state reactivity may well provide a partial explanation for the failure to isolate this compound as a stable species.
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