Statistical and molecular mechanics analysis of the effects of changing donor type on bond length in the two series [Co(III)N(n)O(6-n)] and [Ni(II)N(n)O(6-n)] (n = 0-6): a new route to bond-stretch Parameters

Changes in bond lengths across the series of complexes Co(III)N(n)O(6-n)] and Ni(II)N(n)O(6-n)], (n = 0-6) have been examined by a statistical analysis of the bond lengths in 256 Co(III) and 205 Ni(II) complexes. In both cases a systematic reduction in both metal-N and metal-O bond lengths is observed as oxygen donors replace nitrogen in the coordination sphere. In the case of Co(III), the reduction in bond lengths is linear across the series, whereas, in the case of Ni(II), it is more asymptotic in nature. It was found that this systematic change to the inner coordination sphere produced a much larger range of bond lengths than had previously been observed by changes to the outer sphere. The trends across the two series were reproduced using molecular mechanics; however, the magnitude of the change was not initially predicted correctly in either case. Alterations to molecular mechanics parameters that reproduced the trends in the Co(III)N(n)O(6-n)] series also resulted in a significant overall improvement in the predictions of Co(III)-N bond lengths in a series of Co(III) hexaamines, with all being reproduced within 0.006 A. This improvement was taken as an indication that the bond-length reduction across the series is largely steric in origin.