In the Classroom
Gold Chemistry: The Aurophilic Attraction Manuel Bardají and Antonio Laguna* Departamento de Química Inorgánica, Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
Gold is a noble metal of reddish yellow color. It is soft, ductile, malleable, and a good conductor of heat and electricity. It has been present in man’s life since the earliest civilizations and has occupied an important place in history for more than 7000 years, as is evident in the excellent examples of the goldsmith’s craft found in Minoic, Egyptian, and South American tombs. Platinum, rhodium, and osmium are less abundant and more expensive, but gold has always been considered the king of the elements. This is probably because of its different color and its noble character, which prevents gold coins, jewelry, and electrical conductors for electronics from tarnishing, even after long exposure to extremely aggressive conditions. Although gold metal has been omnipresent in human history, its chemistry has played a minor role, usually relating to the concentration, recovery, and purification of the metal (1). Chemically, there are some unusual facts about gold: 1. Its electrochemical potential is the lowest of any metal, which means that gold in any cationic form will accept electrons from virtually any reducing agent to form metallic gold. 2. Gold is the most electronegative of all metals (in the Pauling or absolute scales but not in the Allred– Rochow scale), which confirms again its noble character (2). 3. Gold is very easily reduced to Au᎑ ; the anionic derivative Cs +Au᎑ has been known since 1930. 4. Gold vapor consists of diatomic molecules whose dissociation energy is 221 kJ/mol, higher than that of many other diatomic nonmetal elements, such as iodine (3). 5. Many gold compounds show a strange interatomic attractive force which, weak though it is, seems to determine many structural properties.
The term aurophilicity or aurophilic attraction was coined by Schmidbaur (4 ) to refer to this empirically found phenomenon, which involves the attraction between gold atoms (and it is not a fatal attraction for gold, which is a different disease!). The phenomenon already appears in gold metal, in which the gold–gold distance is even shorter than the corresponding silver–silver distance (2.884 versus 2.889 Å). Although this is a general fact when comparing radii of the second- and the third-row elements (for instance the metallic radii of Cd and Hg are both 1.51 Å), it cannot be considered simply as a consequence of the “lanthanoid contraction”, because it cannot explain all these gold anomalies. It also appears in gold clusters, where the gold centers are in mixed *Email:
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oxidation states between zero and one, in other gold(I) and gold(III) complexes, and in the newly discovered gold(II) species. Recently it has been found experimentally (5) that for two-coordinate Au(I) compounds the radius is 1.25 Å, whereas for Ag(I) it is 1.33 Å; and for four-coordinate Au(I) the radius is 1.37 Å, whereas for Ag(I) it is 1.46 Å. Copper– copper distances as short as 2.35 Å have been reported for Cu(I) complexes (copper centers are brought close by bridging ligands); and recently the term “cuprophilicity” has been used for a complex with an unsupported Cu–Cu distance of 2.81 Å—therefore without bridging ligands that stereochemically can force the proximity of the metal atoms (6 ). Also, some short metal–metal distances have been found in compounds of other d10 ions. The term aurophilic attraction was first coined in relation to gold(I) chemistry. The electronic configuration for gold(I) is 5d106s0, a closed-shell configuration; however, X-ray diffraction studies show that some gold(I) molecules are often associated in dimers, trimers, chains, or even layers, in such a way that gold–gold distances are 2.75–3.60 Å (the estimated Van der Waals distance being 3.60 Å). Hundreds of examples can be found in the Cambridge Crystallographic Database. Values from 20 to 50 kJ/mol, close to those found in hydrogen bonds, have been obtained for these AuI–Au I interactions by making temperature-dependent NMR measurements (7 ). These measurements are always carried out in dinuclear gold(I) complexes attached by a bidentate ligand, which can exist in different conformations. For instance, the diphosphine Ph2PC(=PMe2)PPh2 shows a syn/anti orientation of the lone pairs both in solution and in the crystalline state, and requires 40 kJ/mol to change to a syn/syn, as has been determined by means of a 31P-NMR variable-temperature experiment (syn/anti conformer shows three different phosphorus, whereas syn/syn shows only two). After double complexation to Au–Cl, the ground state (solution and solid state) is now the symmetrical syn/syn conformation, which allows the gold–gold interaction. From the previous data and a similar 31P-NMR variable temperature experiment, it has been determined that the energy of this interaction is on the order of 29–33 kJ/mol (Scheme I).
Scheme I
As we can see in Scheme II, these contacts appear in nonbridged (a, b), mono-bridged by one or more donor atoms (c, d), two-bridged dinuclear derivatives (e), and more complicated polynuclear systems (for instance, f ).
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In the Classroom
the post-lanthanide elements whose atomic mass increases and whose orbital radius decreases. Because all orbitals must be orthogonal to one another, a similar orbital contraction occurs for 2s, 3s, 4s, 5s, and 6s. Figure 1 shows a plot of the ratio of the 6s orbital radii using relativistic and nonrelativistic calculations versus the atomic number Z. A pronounced local minimum is shown for gold, which means that gold occupies a unique position among the elements. Besides, 5d orbitals are slightly expanded and destabilized because of the increased shielding of the nucleus by the s electrons. The consequences of this effect in gold chemistry are as follows:
Scheme II
The presence of these interactions is more evident in non-bridged complexes of stoichiometry AuXL, in which the gold(I) center is dicoordinated in a linear configuration and, moreover, contacts with the gold center of the next molecule to form dimers, trimers, or chains. Distances from 2.88 Å have been found, depending on the nature of ligand— basically, on its steric requirements (8). Case c with a single atom bridge has produced the most striking derivatives. Complexes containing Cl, O, S, N, P, As, C, or B as bridge have been obtained. In Table 1 we summarize the main features of some of them, including some amazing cases of hypercoordination at oxygen, sulfur, nitrogen, phosphorus, or carbon—unknown derivatives in the typical p-block chemistry, although some of them are postulated as unstable intermediates. The aurophilic attraction seems to play a role by stabilizing these complexes, which always show short–short gold–gold distances. In some cases it plays a major role by determining the geometry—for instance, for [S(AuPPh3)4]2+, which adopts a pyramidal structure (with shorter gold–gold distances) instead of a tetrahedral one. Cases d and e provide the shortest gold–gold distances, like the ones found in [Au2(µ-S 2CNnPr2) 2] (2.76 Å intramolecular, 3.40 Å intermolecular). By changing the nature of the bridging ligands the strength of intramolecular and intermolecular interactions can be tuned, from very high even to disappearance, as found in the series [Au2(µ-S2CNEt 2) 2] (2.782 Å intramolecular, 3.004 Å intermolecular), [Au2(µ(CH 2) 2PPh 2 )(µ-S 2 CNEt 2 )] (2.8665 Å intramolecular, 2.9839 Å intermolecular), or in [Au 2(µ-(CH 2) 2 PPh 2) 2 ] (2.977 Å intramolecular, discrete dinuclear species) (16 ). Physicists and theoretical chemists first tried to explain this striking phenomenon by using the extended Hückel theory, and they proposed an s/d hybridization; this explanation was reinforced by the relativistic approach of the 5d and 6s gold orbitals. Then, ab initio and functional density calculations refuted the previous explanation (because there is no attraction at Hartree–Fock level) and suggested an electronic correlation effect; again, it was found that this attraction is considerably strengthened by the relativistic effect (17 ). In the post-lanthanide elements the inner electrons move in a field created by a very high nuclear charge (79 protons for gold), which leads to velocities approaching light velocity; therefore they have to be treated according to Einstein’s theory of relativity. This has a marked effect on the 1s electron of 202
1. The contraction of the 6s orbitals results in stabilization of these electrons, which are more difficult to ionize; in consequence, it is harder to obtain cations. This can also explain the high electron affinity and the possibility to accept electrons to give anions. 2. The energy gap between 5d, 6s, and 6p orbitals diminishes, the closed shell configuration 5d10 is no longer chemically inert, and the interaction between two gold(I) centers with equal charges can be explained. That also favors formation of linear two-coordinated gold(I) complexes. 3. The destabilization of the 5d orbitals explains the ease of formation of gold(III), almost absent in copper and silver.
Table 1. Some Gold(I) Complexes of Type c Compound
Geometry
Cl[AuP(C6H5)3]2+
angular
O[AuP(o-Tol)3]42+ a
tetrahedral
10
S[AuP(C6H5)3]42+
square pyramidal
11
N[AuP(C6H5)3]5
trigonal bipyramidal
12
P[AuP(C6H5)3]52+
square pyramidal attached to an apical AuP(C6H5)3
13
C[AuP(C6H5)3]5+
trigonal bipyramidal
14
C[AuP(C6H5)3]62+
octahedral
15
2+
ao -Tol
Reference
9
is o -tolyl, C7 H7.
Figure 1. Relativistic contraction of 6s orbitals for heavy elements of atomic number Z = 70 (Yb) to 90 (Th). Adapted from Pyykkö (17a, 18 ). Ordinate: calculated 6s orbital relativistic radius vs calculated 6s orbital nonrelativistic radius (the numbers are taken from the Dirac and Hartree–Fock calculations of Desclaux, ref 19 ).
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In the Classroom
Relativity has also been used to explain other striking anomalies found in the third-row elements and the actinoids (17a): for instance, the facts that mercury is a liquid at room temperature and is able to amalgamate with Cu, Ag, Au, and the alkali metals (20). The latest studies on this phenomenon carried out by Pyykkö and coworkers (21) have concluded that the “metallophilic attraction” is an electronic correlation effect strengthened by the relativistic effect, which has a maximum for gold. Therefore they have extended the studies to other closed-shell interactions, d10–d10 in CuI, AgI, HgII, Pd0, or Pt0, s2–s2 in Hg2, Tl I, or PbII; but the gold ones are the strongest. Literature Cited
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