Limitations of VBT

Valence bond theory, despite its usefulness for predicting a complex's geometry and magnetic behaviour, has three well-recognised limitations. First, it CANNOT itself explain why a particular complex turns out to be high spin or low spin -- VBT has to be TOLD in advance (usually from an experimentally measured magnetic moment) whether a given ligand behaves as strong-field or weak-field; it offers no independent, first-principles way to distinguish the two. Second, VBT provides NO explanation at all for the colour of coordination compounds -- it says nothing about why complexes absorb specific wavelengths of visible light. Third, the structure that VBT predicts for a given complex does not always match the structure that is actually observed experimentally. These three gaps are exactly what motivated the development, and subsequent wide acceptance, of Crystal Field Theory (section 9.9.6 onward), which fills in precisely the questions VBT cannot answer -- the origin of high spin/low spin behaviour and of colour …