Primary and Secondary Valences

Werner discarded Kekule s distinction between valence compounds, which are eminently explainable using classical valence theory, and molecular compounds, which are not. Werner proposed a new approach in which the configurations of some compounds— metal-ammines (now sometimes called Werner complexes ), double salts, and metal salt hydrates— were logical consequences of their coordination numbers (a new concept) and two types of valence, primary and secondary. For compounds having coordination number six he postulated an octahedral configuration for those having coordination number four he proposed a square planar or tetrahedral configuration. 

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