Changes in bond lengths across the series of complexes [(CoNnO6-n)-N-III] and [(NiNnO6-n)-N-II], (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 [(CoNnO6-n)-N-III] 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 Angstrom. This improvement was taken as an indication that the bond-length reduction across the series is largely steric in origin.