Aurophilic effect

Other notable interactions unique to organometallic systems include metal-metal interactions such as those between gold atoms - the aurophilic effect . The strength of these Au Au interactions has been estimated to be in... 

The trigonal bipyramidal and octahedral structures are thought to be a consequence of an aurophilic effect, that is, there are attractive Au—Au interactions which are thought [58] to be due to a combination of relativistic and electron correlation effects. 

Scherbaum, F., Grohmann, A., Huber, B., Kruger, C. and Schmidbaur, H. (1988) Aurophilicity as a Consequence of Relativistic Effects The Hexakis (triphenylphosphaneaurio)methane Dication [(Ph3PAu)6C]2. Angewandte Chemie International Edition, 27, 1544-1546. 

Pyykkd, P. and Zhao, Y.-F. (1991) Ab initio calculations on the (ClAuPH )2 dimer with relativistic pseudopotential Is the aurophilic attraction a correlation effect Angewandte Chemie International Edition, 30, 604—605. 

Rosch, N., Gorling, A., Ellis, D.E. and Schmidbaur, H. (1989) Aurophilicity as Concerted Effect Relativistic MO Calculations on Carbon-Centered Gold Clusters. Angewandte Chemie International Edition, 28, 1357-1359. 

Fig. 1 Schematic molecular orbital diagram showing the effect of aurophilic contacts on the HOMO-LUMO energy gap of gold(I) phosphine complexes...

As described above, among the several closed-shell metal ions that form luminescent supramolecular entities with gold, thallium(I) forms the most numerous examples. While aurophilic attractions can be considered the upper extreme of the metallophilic interactions (with values up to 46 kJ mol-1), intermetallic contacts involving T1(I) centers appear as the weakest ones (even <20 kJ mol-1),46 which is explained by the enhancement of the Au---Au interactions and the weakening of the Van der Waals attractions between the s2 metal atoms produced by the relativistic effects.47 Nevertheless, the complexes in which this interaction appears are surprisingly stable, with additional electrostatic, packing forces, or the ligand architecture, responsible for this fact. 

Aurophilicity presumably arises from relativistic modification of the gold valence AOs energies, which brings the 5d and 6s orbitals into close proximity in the energy level diagram. In more recent theoretical studies, the effect is primarily attributed to electron correlation, which takes precedence over 6s/5d hybridization. To date, the origin of the aurophilicity has not yet been unambiguously established. 

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