Covalent bonds one-electron

It remained for G.N. Lewis (2), M.L. Huggins (3), and their contemporaries to interpret the primary and secondary valences of Werner in terms of the newly emerging electron patterns which were being used to explain "valence." Primary valences were normal covalent bonds (one electron from each bonded atom) and secondary valences were coordinate covalent bonds (two electrons for bond formation supplied by the ligand). The terms Lewis base and Lewis acid entered the literature. The Werner and Lewis-Huggins descriptions of stmcture and bonding were excellent for the metal-ammines, halo complexes of metals, and related species. Classical Werner coordination compounds fit the Wemer-Lewis description, and there were many of such compounds. 

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