The δ Bond and Trigonal Paddlewheels Before the Dawn of the Quintuple Bond

Abstract

Recognition five decades ago of the existence of a quadruple bond in the (Science 1964, 145, 1305) brought the δ bond into the toolbox of inorganic chemists. This discovery was followed by an explosive growth of the field of metal–metal bonding that led to significant advances in basic chemical knowledge which have transcended the curiosity stage. A few examples are provided here to illustrate how the study of species with a δ bond has contributed to improved understanding of electronic communication, the observation of steps that led to formation of chemical bonds as well as the syntheses of the most easily ionized, isolable chemically stable molecules having closed-shells. Many of these quadruple-bonded molecules have the well-known paddlewheel structure with four bridging ligands spanning two metal units. However, there are also a few dimetal species with only three bridging ligands. An account is given of some aspects of the early work on these trigonal paddlewheels with metal–metal bonds which are now an important motif in some quintuple-bonded species that have two δ bonds.