The Origin of the Sigma, Pi, Delta Notation for Chemical Bonds William B. Jensen Journal of Chemical Education 2013 90 (6), 802–803 دُوِي:10.1021/ed200298h
Bonding Power of Electrons and Theory of Valence. Robert S. Mulliken Chemical Reviews 1931 9 (3), 347–388 دُوِي:10.1021/cr60034a001
Metal-Metal Bonding in [Re2X8]2− Ions and Other Metal Atom Clusters F. A. Cotton Inorganic Chemistry 1965 4 (3), 334–336 دُوِي:10.1021/ic50025a016
