Conformational analysis of coordination compounds. VII. Barrier to inversion of five-membered diamine chelate rings by N.M.R

Abstract

The barriers to δ ↔ λ interconversion for the five-membered diamine chelate rings in [M(CO)₄(tmen)], where M is chromium(0), molybdenum(0), or tungsten(0) and tmen is N,N,N',N'-tetramethylethane-1,2-diamine, have been determined to be of the order of 39 kJ mol^-1 from a band-shape analysis of ¹³C N.M.R. spectra of the compounds at low temperature. The results have been rationalized in terms of the effects of M-N bond lengths on ring strain energies and have been compared with the barriers determined previously for analogous complexes with coordination numbers of four (tetrahedral) and eight. The equivalent six-membered diamine chelate ring in [Mo(CO)₄(tmtn)] was found to have a relatively low barrier to chair-to-chair interconversion with an upper limit of 34 kJ mol^-1.