Variable temperature 1 H NMR studies were conducted on isolated samples of 2b dissolved in toluened . Data were recorded across a -80 to +80 °C temperature range and showed that the equilibrium posiOon shiis enOrely toward 2b at lower temperatures. A van't Hoff analysis is consistent with the forward reacOon having a Gibbs free energy of DG°298K = -4.7 kcal mol -1 . The process is reversible, as confirmed by a cross-over experiment between 2b and 1a which resulted in complete conversion to 2a and 1b. 2a-c could be crystallised and isolated in 45 -55 % yield. In the solid-state, 2a-c all demonstrated an approximate tetrahedral geometry at the Al, Ga, or Si centres (Figure ). The Zn---Zn distances are all in excess of 4 Å. These distances are beyond the sum of the covalent radii (Pyykkö, 2.36 Å; Pauling, 2.50 Å ) and are far greater than those experimentally determined in coordinaOon complexes of Cp*ZnZnCp* with transiOon metal fragments which range from approximately 2.4 to 2.8 Å. The data suggest that complete breaking of the Zn-Zn bond occurs on reacOon with the main group experimentally determined structures. Intrinsic Bonding Orbital (IBO) analysis on 2a-c returns clear sbonding orbitals between the central main group element and zinc with coefficients approximately evenly distributed between the bonding atoms (Figure ). Natural Bonding Orbital (NBO) analysis also supports the importance of a primarily covalent bonding interacOon with Wiberg Bond Indicies (WBIs) taking average values of 0.84, 0.79, and 0.68 for the E-Zn bonds of 2a, 2b and 2c respecOvely. Atoms in Molecules (AIM) analysis returned bond paths and associated bond criOcal points (BCPs) from the central main group element to each of the zinc atoms in 2a-c, with no bond paths between the two zinc atoms themselves (Figure ). Metrics associated with the (BCPs) are consistent with their descripOon as weak covalent bonds. Independent Gradient Model based on Hirshfield parOOon (IGMH) analysis also supports an aYracOve covalent interacOon with no or liYle Zn---Zn bonding (Figure ). In combinaOon the calculaOons support the proposal that complete Zn-Zn bond breaking occurs on addiOon to 1a-c and these species are best formulated as tetrahedral four-coordinate complexes with Zn sites covalently bonding to the central atom. Pathways for the addiOon of Cp*ZnZnCp* to 1a-c were calculated by DFT (Figure ). 1a-c can all be considered low-valent complexes; each has an available lone-pair. In all cases, the reacOon is proposed to occur through formaOon of an intermediate Int-1, derived from coordinaOon of the lowvalent fragment to one of the Zn centres of Cp*ZnZnCp*. This coordinaOon event is endergonic for both Si and Ga (DG°298K = +4.8 to +9.7 kcal mol -1 ) but slightly exergonic for Al (DG°298K = -0.8 kcal mol - addiOon transiOon state, TS-1. TS-1 is a low energy (DG ‡ 298K = 2.3 to 17.5 kcal mol -1 ) three-centred transiOon state, reminiscent of those found in oxidaOve addiOon mechanisms for both main group and transiOon metal complexes (Figure ). TS-1 appears late in the reacOon pathway, with Zn-Zn bond breaking and Zn-E bond breaking almost complete as it is traversed. While formaOon of 2a-c are all calculated to be downhill, the Ga analogue is the least exergonic being only DG°298K = -5.5 kcal mol -1 downhill, consistent with the potenOal for this reacOon to be reversible within the expected accuracy of the calculaOons.