Argento-aurophilic bonding in organosulfur complexes. The molecular and electronic structures of the heterobimetallic complex AgAu(MTP)2

Article abstract of DOI:10.1016/S0020-1693(02)00864-2

The heterobimetallic complex AgAu(MTP)₂, (1), was obtained from the reaction of PPN[Au(MTP)₂] with AgNO₃ (MTP=diphenylmethylenethiophosphinate; PPN=bis(triphenyl-phosphoranylidene) ammonium). Compound 1 was found to be isomorphous with but not isostructural to the previously reported compounds Au₂(MTP)₂, (2) and Ag₂(MTP)₂, (3). Surprisingly, 1 has a very short intramolecular Ag-Au distance (2.912 ?), which is much shorter than the intramolecular metal-metal distances in 2 and 3 (～3.0 ? in both). The three compounds form extended one-dimensional chain structures in the solid state. The intermolecular interactions in 1 were found to be Ag-Au interactions (3.635 ?), as opposed to Au-Au and Ag-Ag interactions. Density-functional theory (DFT) calculations were used to study the stability of the geometrical isomers with different coordination modes of the MTP ligand in models of 1-3. Isomers with Ag-C bonds were found much higher in energy than those with Au-C bonds, which explains the stability of 1 and 2, which have Au-C bonds, relative to 3, which has two Ag-C bonds. Dilute solutions of the three compounds showed virtually identical absorption spectra in which the lowest-energy band is due to a π-π* intraligand transition. The electronic spectra for concentrated solutions and the solid state show lower energy bands due to intermolecular interactions. The relative energies of the absorption edge followed the order Au₂AgAu2. DFT calculations demonstrate that monomer models cannot describe this trend. However, DFT calculations for dimer and trimer models of the three compounds give excellent agreement with the experimental results, as the HOMO-LUMO gap follows the same order as the absorption edge. The importance of intermolecular metal-metal interactions is further manifested by the presence of emission bands in the visible region for the three compounds. Crystal data for AgAu(CH ₂P(S)Ph₂)₂: monoclinic, space group C2/c; Z=8; a=24.384(2) ?; b=6.5472(6) ?; c=16.4884(18) ?; β=113.356(5)°.

Full text of DOI:10.1016/S0020-1693(02)00864-2

Inorganica Chimica Acta 334 (2002) 376Á384
/
www.elsevier.com/locate/ica
ArgentoÁ
/
aurophilic bonding in organosulfur complexes. The
molecular and electronic structures of the heterobimetallic complex
AgAu(MTP)2
Manal A. Rawashdeh-Omary a, Mohammad A. Omary b,*, John P. Fackler, Jr. a,*
a Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
b Department of Chemistry, University of North Texas, Denton, TX 76203, USA
Received 11 December 2001; accepted 25 January 2002
It is a pleasure to help celebrate the career of Professor Andrew Wojcicki, a long time friend and a superb organometallic chemist.
Abstract
The heterobimetallic complex AgAu(MTP)2, (1), was obtained from the reaction of PPN[Au(MTP)2] with AgNO3 (MTPꢀ
diphenylmethylenethiophosphinate; PPNꢀbis(triphenyl-phosphoranylidene)ammonium). Compound 1 was found to be isomor-
phous with but not isostructural to the previously reported compounds Au2(MTP)2, (2) and Ag2(MTP)2, (3). Surprisingly, 1 has a
/
/
˚
very short intramolecular AgÃ
/
Au distance (2.912 A), which is much shorter than the intramolecular metalÁ
/
metal distances in 2 and
˚
3 (ꢀ
interactions in 1 were found to be AgÃ
theory (DFT) calculations were used to study the stability of the geometrical isomers with different coordination modes of the MTP
ligand in models of 1Á3. Isomers with AgÃC bonds were found much higher in energy than those with AuÃC bonds, which explains
the stability of 1 and 2, which have AuÃC bonds, relative to 3, which has two AgÃC bonds. Dilute solutions of the three compounds
showed virtually identical absorption spectra in which the lowest-energy band is due to a pÁp* intraligand transition. The electronic
spectra for concentrated solutions and the solid state show lower energy bands due to intermolecular interactions. The relative
energies of the absorption edge followed the order Au2ꢁAgAuBAg2. DFT calculations demonstrate that monomer models cannot
describe this trend. However, DFT calculations for dimer and trimer models of the three compounds give excellent agreement with
the experimental results, as the HOMOÁLUMO gap follows the same order as the absorption edge. The importance of
intermolecular metalÁmetal interactions is further manifested by the presence of emission bands in the visible region for the three
compounds. Crystal data for AgAu(CH2P(S)Ph2)2: monoclinic, space group C2/c; Zꢀ
/
3.0 A in both). The three compounds form extended one-dimensional chain structures in the solid state. The intermolecular
˚
/
Au interactions (3.635 A), as opposed to AuÃ
/
Au and AgÃ
/
Ag interactions. Density-functional
/
/
/
/
/
/
/
/
/
/
˚
˚
/
8; aꢀ
/
24.384(2) A; bꢀ
/
6.5472(6) A; cꢀ
/
˚
16.4884(18) A; bꢀ
/
113.356(5)8. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Crystal structures; Electronic structures; Silver complexes; Gold complexes; Bimetallic complexes
1. Introduction
has been presented recently based on Raman spectro-
scopy, which showed AgÃAg vibrations that correlate
with the short AgÃAg distances in the crystal structures
of mononuclear [3] and binuclear [4] Ag(I) compounds.
Closed-shell interactions in homometallic complexes of
Cu(I) [2,5] are also known, but this has been based
/
Coordination compounds of gold(I) are well known
to have aurophilic bonding, namely closed-shell (d10Ád10)
interactions between monovalent gold atoms. This
phenomenon has been observed in a variety of mono-
nuclear and multinuclear Au(I) compounds and is
described in both experimental and theoretical reviews
[1,2]. Evidence for an analogous argentophilic bonding
/
/
mostly on structural studies of CuÃ
/
Cu distances without
evidence based on vibrational and theoretical studies
that might show a bonding character for the Cu(I)Ã
/
Cu(I) interactions [6]. On the other hand, mixed-metal
heterometallic complexes of Group 11 elements are very
* Corresponding authors. Tel.: ꢁ
3154.
/
1-979-845 0648; fax: ꢁ1-979-845
/
scarce [7,8]. A meaningful comparison of d10Ád10
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 8 6 4 - 2

M.A. Rawashdeh-Omary et al. / Inorganica Chimica Acta 334 (2002) 376Á
/
384
377
interactions between complexes of different metals
requires that the metals are coordinated to identical
ligands and have identical coordination number, geo-
metry, counterions (in ionic complexes), and space
group. The rare satisfaction of these conditions in
isomorphous Au(I) and Ag(I) compounds has allowed
Schmidbaur and co-workers to discover that gold is
smaller than silver, contrary to what was believed prior
electronic structure is paramount, as suggested from
the electronic spectra and semi-empirical calculations
for coordination compounds possessing such a struc-
ture. Here we demonstrate this concept by modern
computational methods based on density-functional
theory (DFT) calculations for models containing up to
six metal ions, the results of which were correlated with
the experimental spectroscopic results for the three
to these studies [9]. Here we present a study of Au(I)Ã
Au(I), Ag(I)ÃAg(I), and Ag(I)ÃAu(I) bonding in three
isomorphous binuclear compounds of the same ligand.
Synthetic, structural, and spectroscopic characteriza-
tion of organosulfur complexes of transition metals,
especially those with a closed-shell electronic configura-
tion has long been studied in the Fackler group [10,11].
Included in this work has been the synthesis of homo-
and hetero-bimetallic and trimetallic complexes of the
/
isomorphous bimetallic complexes 1Á3.
/
/
/
We report here the synthesis, crystal structure, and
electronic absorption and luminescence spectra of the
new heterobimetallic complex AgAu(MTP)2, (1). The
molecular and electronic structures are compared with
the related homobimetallic complexes Au2(MTP)2, (2)
and Ag2(MTP)2, (3). The intramolecular and intermo-
lecular metalÃ/metal bonding in the three compounds
are compared based on X-ray crystallography and DFT
calculations. The various coordination possibilities, for
the MTP ligand to the Ag and Au atoms, in models of
the three compounds are analyzed based on their DFT
energies. Finally, the absorption and luminescence
properties for solutions and solids of the three com-
plexes are studied and related to the extended-chain
structure.
ligand
diphenylmethylenethiophosphinate
(MTP),
which coordinates to the metal atom through sulfur
and/or carbon [10]. To date, a significant number of
transition metal complexes of the MTP ligand have been
synthesized and characterized by X-ray crystallography
[10Á
complexes of Au(I) and Ag(I). The homobimetallic
complexes M2(MTP)2 (MꢀAu; Ag) have been reported
/
13]. An important subset of this class includes
/
earlier [12,13]. Heterobimetallic complexes of MTP with
two different monovalent coinage metal ions were
previously unknown. The reported heterobimetallic
and heterotrimetallic complexes of MTP contain Au(I)
with the ions Pt(II), Tl(I), Pb(II), or Hg(II) [10,11].
An important feature of many of the MTP complexes
of closed-shell metals is their extended linear chain
structures. Linear chain transition metal complexes have
attracted the interest of experimental and theoretical
chemists and physicists for many years due to their
fascinating chemical and photophysical properties
[14,15]. Recent examples include the development of a
vapochromic light emitting diode from a linear chain
Pt(II) complex, [16] the synthesis of an Au(I) dithiocar-
bamate complex that possesses a luminescent linear
chain structure only in the presence of vapors of organic
solvents, [17] and a trinuclear Au(I) complex whose
extended chain structure is responsible for its storage of
energy and releasing it as long-lived orange phosphor-
escence, ‘solvoluminescence’, upon contact with solvent
[18]. We have recently reported that colorless trinuclear
Au(I) compounds that do not form extended-chain
structure by themselves can produce brightly-colored
complexes by sandwiching naked Tlꢁ and Agꢁ ions to
form linear chain complexes with fascinating lumines-
cence properties that include luminescence thermochro-
mism [7]. Recent results have demonstrated that the
electron-rich trinuclear Au(I) complexes can interact
with neutral inorganic [19] and organic [20] Lewis acids
to produce infinite linear chain complexes. The influence
of the extended-chain molecular structure on the
2. Experimental
2.1. General procedures
All reactions were carried out under a nitrogen
atmosphere using standard Schlenk techniques [21].
All solvents were freshly distilled and de-aerated prior
to use. PPN[Au(MTP)2], Au2(MTP)2 and Ag2(MTP)2
were prepared according to published procedures [10Á
13]. AgNO3 and PPNCl were purchased from Aldrich
/
and used without further purification.
1H and 31P NMR measurements were carried out
using a Unity Plus 300 spectrometer operating at 300
MHz for proton spectra. Absorption spectra were
carried out using a HewlettÁ
/
Packard model HP 8452
UVÁVis spectrophotometer. The absorption spectra
/
were obtained for solutions of the compounds in
freshly-distilled reagent grade CH2Cl2. Luminescence
spectra were obtained on an SLM/AMINCO Model
8100 spectrofluorometer using a 150 W xenon lamp.
Low temperature luminescence measurements were
made by placing crystalline solids in a cryostat of local
design. Liquid nitrogen was used to obtain the 77 K
data.
2.2. Synthesis of AgAu(MTP)2 (1)
A 390 mg sample of PPN[Au(MTP)2] (0.33 mmol)
was dissolved in 10 ml of CH2Cl2. To this CH2Cl2
solution, 55 mg of AgNO3 (0.33 mmol) were added. A

378
M.A. Rawashdeh-Omary et al. / Inorganica Chimica Acta 334 (2002) 376Á384
/
white precipitate started to form after 15 min of stirring
at ambient temperature. The mixture was stirred for 1 h
and then the solid was filtered and washed with CH3OH
and diethyl ether. The product was dried under reduced
pressure for 1 h and then weighed (90% yield). Com-
pound 1 was recrystallized from CH2Cl2 at ꢂ20 8C.
/
The white solid is stable in air and room light and has a
melting point (m.p.) of 165 8C (dec.). 1H NMR
(CDCl3Á
/
TMS): 1.80 ppm, d, CH2, JPÃH
ꢀ
/
12 Hz; dꢀ
/
7.3Á
/
7.9 ppm, m, C6H5. 31P{1H} NMR 52.17 ppm
relative to a 85% H3PO4 in D2O standard. No AgÃ
/
P
Scheme 1. Structures of 1, 2, and 3. The R groups are phenyl groups in
the crystal structures. In the molecular models used, the R groups were
phenyl, methyl, and hydrogen, depending on the calculations type
(text).
coupling was detected by 31P NMR even at ꢂ
(CDCl3) or in the solid-state. Calc.
/
80 8C
for
C26H24P2S2AuAg: C, 40.70; H, 3.15; S, 8.36%. Found:
C, 40.41; H, 3.07; S, 8.25%.
2.3. Crystal structure determination of 1 by X-ray
diffraction
Single crystals of 1 were grown by recrystallizing a
warm CH2Cl2 solution at ꢂ
crystals that proved suitable for X-ray crystallography
were obtained after ꢀ24 h of storage at ꢂ20 8C. The
structure was determined using direct methods (SHELXL
/
20 8C. Good quality block
/
/
Scheme 2. Alternative bonding modes considered in the molecular
structure models of 1, 2, and 3.
-
97, Sheldrick, 1997). Details of the crystal data, para-
meters for data collection, the solution and refinement
of the structure are given in Table 1.
hybrid exchange functional and the LeeÁ
/
YangÁParr
/
correlation functional (B3LYP) [24]. A Huzinaga/Dun-
ning basis set [25] of a double-zeta quality was used for
carbon and hydrogen atoms. A double-zeta basis set
plus one polarization function on the P and S atoms was
used, in order to properly describe the hypervalent
character of phosphorous and sulfur compounds. A
small-core effective core potential (ECP) developed by
Hay and Wadt [26] was used for gold atoms to represent
the 60 core electrons (1s2 2s2 2p6. . . 4d10 4f14) with a
double-zeta basis set for the 19 outer electrons from the
atomic 5s2 5p6 5d10 6s1 shells. The ECP for gold
incorporates two relativistic effects for the core elec-
trons, mass velocity and Darwin, and thus represents the
dominant relativistic contributions to the behavior of
the outer electrons. Single point energy and geometry
optimization calculations were carried out on the model
systems with geometries taken directly from their X-ray
crystal structures.
2.4. Computational details
All calculations were performed with the GAUSSIAN
98 suite of programs for molecular models of 1, 2, and 3
shown in Schemes 1 and 2. The calculations were at the
DFT level, [22] using the Becke three parameter [23]
Table 1
Crystal data and structure refinement for AgAu(MTP)2
Chemical formula
Formula weight
Crystal system
Space group
C13H12Ag0.50Au0.50PS
383.67
monoclinic
C2/c
110(2)
Temperature (K)
˚
Wavelength (A)
0.71073
Unit cell dimensions
˚
a (A)
˚
24.384(2)
6.5472(6)
16.4884(18)
113.356(5)
2416.6(4)
8
b (A)
˚
c (A)
b (8)
3
V (A )
˚
3. Results and discussion
Z
rcalc (mg mꢂ3
)
2.109
7.193
m (mmꢂ1
)
3.1. Molecular structure
a
R1 [I ꢂ2s(I)]
wR2 [I ꢂ2s(I)]
R1 (all data)
0.0450
0.1077
b
Crystallographic data for 1 are shown in Table 1. The
molecular structure of the unit cell of 1 is shown in Fig.
1. The Ag and Au atoms are bridged by the MTP ligand.
The Au atom is coordinated linearly to two carbon
a
0.0572
0.1155
b
wR2 (all data)
a
R1ꢀajjFojꢂFcjj/jFoj.
b
wR2ꢀ{a[w(Fo2ꢂFc2)2]/a[w(Fo2)2]}1/2
.
atoms; CÃ
/
AuÃ
/
Cꢀ179.1(6)8. On the other hand, the Ag
/

M.A. Rawashdeh-Omary et al. / Inorganica Chimica Acta 334 (2002) 376Á
/
384
379
Table 2
˚
Selected bond lengths (A) and angles (8) for AgAu(MTP)2
Bond lengths
Ag(1)ÃS(1)
Ag(1)ÃAu(1)
Au(1)ÃC(1)
P(1)ÃC(1)
P(1)ÃC(8)
P(1)ÃC(2)
P(1)ÃS(1)
2.438(3)
2.9124(13)
2.069(13)
1.743(11)
1.788(13)
1.805(11)
2.005(4)
Bond angles
S(1)ÃAg(1)ÃS(1)
S(1)ÃAg(1)ÃAu(1)
C(1)ÃAu(1)ÃC(1)
a
171.71(13)
94.14(7)
179.1(6)
90.5(3)
a
C(1)ÃAu(1)ÃAg(1)
C(1)ÃP(1)ÃC(8)
C(1)ÃP(1)ÃC(2)
C(8)ÃP(1)ÃC(2)
C(1)ÃP(1)ÃS(1)
C(8)ÃP(1)ÃS(1)
C(2)ÃP(1)ÃS(1)
P(1)ÃS(1)ÃAg(1)
P(1)ÃC(1)ÃAu(1)
C(3)ÃC(2)ÃC(7)
C(3)ÃC(2)ÃP(1)
C(7)ÃC(2)ÃP(1)
109.7(5)
109.3(5)
103.3(5)
114.4(4)
109.0(4)
110.6(4)
101.25(15)
115.5(6)
118.1(10)
122.8(8)
119.0(9)
Fig. 1. Molecular structure of AgAu(MTP)2 showing 40% thermal
ellipsoids.
atom is coordinated to two sulfur atoms with a
significant deviation from linearity; SÃ
/
AgÃ
/
Sꢀ
/
a
171.71(13)8. The monoclinic space group C2/c deter-
mined for 1 renders the compound isomorphous with
the previously reported compounds Au2(MTP)2, (2) [12]
and Ag2(MTP)2, (3) [13] although the compounds are
not isostructural since 2 and 3 have a molecular center
of symmetry. The coordination of the MTP ligand to the
metal atoms in the three compounds is shown in Scheme
1. Selected interatomic distances and bond angles are
Symmetry transformations used to generate equivalent atoms:
ꢂx, y, ꢂzꢁ1/2.
given in Table 2 for 1. The AuÃ
/
C and AgÃ
2.07 A and 2.44 A in 1 are similar to the correspond-
ing distances in 2 and 3.
The intramolecular AgÃ
distance is surprisingly very short, compared with the
intramolecular metalÃmetal distances in the compounds
3.0 A in both). It is interesting to note that
the deviation from linearity in the SÃAgÃS coordination
is such that the Ag atom is directed toward the Au atom
(Fig. 1), thus suggesting that the intramolecular AgÃAu
/
S distances of
˚
˚
ꢀ
/
˚
Au distance is 2.912 A. This
/
/
˚
2 and 3 (ꢀ
/
/
/
/
bonding in 1 is inherent and little influenced by the
ligand geometry. Fig. 2 shows that 1 packs as an infinite
Fig. 2. The repeat structure of AgAu(MTP)2.
linear chain with intermolecular metalÁ
tions. It is interesting that these metalÁ
tions are AgÃAu intermolecular interactions, as
opposed to possible AuÃAu and AgÃAg interactions.
The intermolecular AgÃAu distance is 3.635 A, appre-
ciably longer than the AuÃAu and AgÃAg intermole-
cular distances in 2 and 3 (about 3.2 A in both). The
AgÃAuꢀ ꢀ ꢀAg and Auꢀ ꢀ ꢀAgÃAu angles are both 1808, a
consequence of the symmetry of the space group.
Similar AgÃAu distances to the ones in 1 were
reported for the non-classical multinuclear compounds
/metal interac-
/metal interac-
and [Au2Ag2(C6F5)4(L)] (Lꢀ
/
neutral weakly-coordi-
/
˚
nated solvent); 2.78Á
/
2.79 A [8]. However, the AuÃAg
/
/
/
interactions in these examples are mostly electrostatic in
nature and are believed to form owing to the strong
Lewis-base character of the neutral and anionic com-
plexes [Au(m-C2,N3-bzim)]3 [19] and Au(C6F5)2ꢂ, [8],
respectively, which interact with the Agꢁ cation. The
intermolecular interactions in both these cases are
˚
/
/
/
˚
/
/
/
aurophilic interactions, not AuÃ
/
Ag interactions. In 1,
on the other hand, the intramolecular and intermole-
{Ag([Au(m-C2,N3-bzim)]3)2}BF4×
/
CH2Cl2; [7] ꢀ2.8 A
/
˚

380
M.A. Rawashdeh-Omary et al. / Inorganica Chimica Acta 334 (2002) 376Á384
/
cular Au(I)Ã
/
Ag(I) interactions are dominantly closed-
and S atoms, model 1c, has an intermediate energy
between the energies for models 1a and 1b. These results
shell metallophilic bonding interactions in a classical
coordination compound and, in this vein, they are
considered quite unusual and unprecedented in the
literature of monovalent coordination compounds of
Group 11. Nevertheless, because 1 is a heterobimetallic
complex with an unsymmetrical metal core, one would
expect some polarity for the molecule. Indeed, DFT
calculations confirm this prediction. Calculations were
suggest that the AuÃ
/
C bond has a much greater stability
than the AgÃC bond. For isomers of the Au2 com-
/
pound, the crystallographically found arrangement, 2a,
is favored by 3.3 kcal/mol over the model 2b. Interest-
ingly, this is not the case for the Ag2 compound, in
which model 3b is favored, albeit very slightly (1.9 kcal/
mol), over the crystallographically observed arrange-
ment, 3a.
performed for molecules of 1Á3 without any approx-
/
imation in the crystal structure (no atoms were omitted;
no changes in the geometry were made). Single-point
energy DFT calculations for the crystal structure of 1
shows that the molecule indeed possesses a dipole
moment of 3.79 D, thus suggesting a significant polar
character. In contrast, the dipole moments for 2 and 3
are 0.0133 and 0.0076 D, essentially zero as expected for
the centrosymmetric structures found. The Mulliken
atomic charge distribution from these calculations
3.2. Electronic structure
Solutions of 1, 2, and 3 in CH2Cl2 have absorption
spectra that are dependent on concentration. Fig. 3
shows the absorption spectra for dilute solutions of 1, 2,
and 3 in CH2Cl2 at ambient temperature. The three
compounds have rather similar absorption spectra, with
a band around 275 nm. The position of the band is
independent of the identities of the metals in the
complex. Therefore, the assignment of this band is not
strongly metal based, because of the large differences in
the energy levels of Ag(I) and Au(I) [29]. The most
reasonable assignment is an intraligand transition. The
shows that the Ag atom in 1 carries a ꢁ
charge while the Au atom carries a higher charge (ꢁ
0.22). Both Au atoms in 2 have an identical charge of ꢁ
0.06 while the Ag atoms in 3 also have an identical
charge of ꢁ0.12, according to DFT calculations. These
complexes have identical coordination at both metal
centers. This is not true for 1. The strength of the AgÃ
/0.15 partial
/
/
/
large extinction coefficient (ꢀ
sistent with an allowed transition of this type such as a
ligand pÁp* transition.
/
104 Mꢂ1 cmꢂ1) is con-
/
Au bond in 1, therefore, is attributed to both metallo-
philic closed-shell interactions and polar interactions.
The symmetrical bonding of the Ag atom to two S
atoms and of the Au atom to two C atoms in 1 is
/
Upon increasing the concentration, a gradual red shift
in the absorption edge was observed for each of the
three compounds. Fig. 4 shows the absorption spectra
for solutions of 1, 2, and 3 in CH2Cl2 at ambient
temperature with concentration near saturation. A 1-
mm quartz cuvette was used for these measurements in
order to attenuate the strong absorbance of concen-
trated solutions by an order of magnitude. The absorp-
tion edge is shifted from the dilute solution absorption
suggestive of the stronger stability of AuÃ
/
C bonds than
C bonds. This result is consistent with the vastly
large number of organogold compounds reported to
date in comparison with organosilver compounds [27]. It
AgÃ
/
is also noted that 1 and 2, both having AuÃ
/
C bonds, are
C bonds,
very stable compounds while 3, which has AgÃ
/
is extremely sensitive to air, light, and temperature, and
decomposes rather easily and rapidly. Modern compu-
tational methods support the structural observations in
band (at ꢀ275 nm) by about 7800, 14 000, and 6500
/
cmꢂ1 for 1, 2, and 3, respectively. The low absorption
energies for concentrated solutions of the Au2 com-
pound, 2, is consistent with the dark-yellow color for the
solid and concentrated solutions of the compound, while
the AgAu and Ag2 compounds are white and have
colorless solutions. The shift of the absorption edge for
each compound to lower energies is indicative of
molecular aggregation as the concentration is increased.
This is consistent with the extended-chain structures
observed for the three solids, whose electronic bands are
positioned at lower energies than those for the saturated
solutions (vide infra).
this and other studies that point to the stability of AuÃ
/C
bonds. The calculations also shed some light on the
possibility for rearrangements in this class of organo-
metallic complexes of Au(I) and Ag(I). A review of the
structural aspects of this topic has been reported
recently [28] but, to our knowledge, no modern theore-
tical studies have been reported.
We have examined the different coordination modes
of the MTP ligand in the three compounds, shown in
Schemes 1 and 2, by DFT calculations. Full geometry
optimization was carried out for each model after
replacing the phenyl groups with hydrogen atoms. The
results are shown in Table 3. The crystallographic
arrangement for the AgAu compound, model 1a, is
favored over the model with the Ag bonded to two
carbon atoms, model 1b, by nearly 16 kcal molꢂ1. The
model having each metal bonded asymmetrically to C
Based upon these data we suggest that compounds 1Á
/
3 exist as discrete oligomers (e.g. dimers and trimers) in
concentrated solutions, as opposed to the infinite chain
structures for the solids. The absence of well-defined
absorption bands in the low energy regions for the
concentrated solutions precludes accurate calculations
for the formation constants of oligomers of each

M.A. Rawashdeh-Omary et al. / Inorganica Chimica Acta 334 (2002) 376Á
/
384
381
Table 3
Summary of DFT Calculations for Fully Optimized Models of 1, 2, and 3
E (kcal molꢂ1
)
MÃM (A) MÃC (A)
MÃS (A)
PÃC
PÃS
˚
˚
˚
Model d (a.u.)
1a
1b
1c
2a
2b
3a
3b
ꢂ395.631 0.0
ꢂ395.606 ꢁ15.9
3.038
3.164
3.156
3.266
3.216
3.131
3.049
2.133
2.195
2.508
2.401
1.801
1.777
2.189 (Ag); 2.103 (Au) 2.513 (Ag) 2.441 (Au) 1.783 (Ag) 1.809 (Au) 2.037 (Ag) 2.056 (Au)
2.045
2.067
ꢂ395.621 ꢁ6.0
ꢂ385.322 0.0
ꢂ385.317 ꢁ3.3
ꢂ405.917 0.0
ꢂ405.92 ꢂ1.9
2.101
2.131
2.446
2.404
1.808
1.797
2.050
2.063
2.182
2.187; 2.190
2.499
2.493; 2.495
1.787
1.769; 1.783
2.045
2.045; 2.049
See Schemes 1 and 2 for the structures of the models used. The models represent the crystal structures but with hydrogen atoms replacing the
phenyl groups.
Fig. 3. Absorption spectra for dilute solutions of AgAu(MTP)2,
Au2(MTP)2, and Ag2(MTP)2.
compound. Such calculations have been reported for
other Au(I) and Ag(I) compounds that show well-
defined oligomer bands [30]. Our focus here, therefore,
will be limited to the qualitative trend showing that the
absorption energies for the oligomers that are dominant
near the saturation points for the three compounds
follow the order 2ꢁ
/
1B3.
/
Fig. 4. Absorption spectra for solutions of AgAu(MTP)2, Au2(MTP)2,
and Ag2(MTP)2 with concentrations near the saturation limit.
DFT calculations have been carried out for monomer,
dimer, and trimer models of 1, 2, and 3. In order to
reduce the computation time, the phenyl groups of the
MTP ligand were replaced by methyl groups in these
calculations. All bond lengths, bond angles, and dihe-
dral angles in each model were maintained as in the
crystal structures of the compounds. The value of the
energy separation between the highest occupied mole-
cular orbital (HOMO) and the lowest unoccupied
three compounds gave a qualitative agreement with the
experimental results. The values of the HOMOÁLUMO
/
gap in Table 4 for dimer and trimer models followed the
same order for the absorption edge for the concentrated
solutions of 1Á
shifts in the absorption energies upon increasing the
concentration of solutions of 1Á3 underscore the
oligomerization of these species owing to closed-shell
d10Ád10 interactions. Similar spectral changes owing to
Au(I)ÃAu(I) and Ag(I)ÃAg(I) interactions have been
/
3 shown in Fig. 4 [31]. The large red
/
molecular orbital (LUMO), i.e. the HOMOÁLUMO
gap, was calculated for each model. The results are
summarized in Table 4.
It is noted that monomer models cannot describe the
experimental trend for the relative energy of the
/
/
/
/
reported in recent years and were attributed to aur-
ophilic and argentophilic bonding, respectively [30]. The
changes in the absorption spectrum of 1 with concentra-
absorption edge, which followed the order Au2ꢁ
/
AgAuBAg2 (Fig. 4). While monomer models for the
/
DFT calculations are not consistent with these trends,
DFT calculations for dimer and trimer models of the
tion are due to Au(I)Ã
/
Ag(I) (as we demonstrated above)
and hence, this represents the first example of such

382
M.A. Rawashdeh-Omary et al. / Inorganica Chimica Acta 334 (2002) 376Á384
/
Table 4
HOMOÁLUMO gaps for monomer, dimer, and trimer models of 1, 2, and 3 according to DFT calculations
/
Model
E (HOMO) (a.u.)
d (LUMO) (a.u.)
HÁ/L gap (a.u.)
HÁ )
L gap (103 cmꢂ1
/
AgAu
2AgAu
3AgAu
1Au2
ꢂ0.2096
ꢂ0.1943
ꢂ0.1873
ꢂ0.2075
ꢂ0.1809
ꢂ0.1691
ꢂ0.2034
ꢂ0.1861
ꢂ0.1786
ꢂ0.0101
ꢂ0.0125
ꢂ0.0150
ꢂ0.0025
ꢂ0.0021
0.0000
0.1995
0.1818
0.1722
0.2050
0.1788
0.1691
0.2030
0.1863
0.1808
43.78
39.90
37.80
44.99
39.24
37.10
44.55
40.89
39.68
2Au2
3Au2
1Ag2
2Ag2
3Ag2
ꢂ0.0004
0.0002
0.0022
The models represent the crystal structures but with methyl groups replacing the phenyl groups. All interatomic distances and bond and dihedral
angles are the same as the crystallographic values.
spectral changes being due to argento Áaurophilic bond-
/
ing. While deviation from Beer’s law due to oligomer-
ization has also been reported recently for
[Au2Ag2(C6F5)4(L)] species, [8c] Au(I)Ã/Au(I) aurophilic
bonding was responsible for those spectral changes. The
calculations suggest this is unlikely to be responsible for
the oligomerizations here for 1.
Compounds 1Á
solid state. Figs. 5Á
/
3 display photoluminescence in the
7 show the luminescence excitation
/
Fig. 7. Emission (thick lines) and excitation (thin lines) of Ag2(MTP)2
in the solid state at 77 K.
and emission spectra of 1Á
state at 77 K. It is noted that the excitation energies for
the solids (Figs. 5Á7) are somewhat lower than the
/3, respectively, in the solid
/
absorption energies of the concentrated solutions (Fig.
4). This is obviously due to the expected lower band gap
energy in the extended-chain species in the solid state, as
opposed to the situation in oligomers of a few complexes
in solution. Intermolecular d10Ád10 bonding is a known
reason for the existence of photoluminescence in linear
two-coordinate complexes of coinage metal monovalent
/
Fig. 5. Emission (thick lines) and excitation (thin lines) spectra of
AgAu(MTP)2 in the solid state (single crystals) at 77 K.
ions [32]. The emission energies are in the blueÁ
/
green
visible region (lmax 424, 493, and 466 nm for 1, 2, and
ꢀ
/
3, respectively). The observed site-selective excitation in
3 is at odds with the presence of only one crystal-
lographically unique Ag site in the compound. Similar
observations have previously been reported and attrib-
uted to the so-called self-trapped excitons, [33] but the
case for 3 has not been established.
4. Conclusions
The present work presents an integrated structural/
theoretical/spectroscopic study of organosulfur coordi-
Fig. 6. Emission (thick lines) and excitation (thin lines) spectra of
Au2(MTP)2 in the solid state at 77 K.

M.A. Rawashdeh-Omary et al. / Inorganica Chimica Acta 334 (2002) 376Á
/
384
383
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underscore the significance of closed-shell, argento Á
aurophilic bonding, the polarity of which has led to
surprisingly short AgÃAu distances.
/
(k) D.D. Heinrich, R.J. Staples, J.P. Fackler, Jr., Inorg. Chim.
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(b) L.C. Porter, J.P. Fackler, Jr., Acta Crystallogr. Sect. C 43
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[13] S. Wang, J.P. Fackler, Jr., T.F. Carlson, Organometallics 9 (1990)
1973.
Acknowledgements
The Robert A. Welch foundation of Houston, Texas,
provided financial support for this research at Texas
A&M University. MAO acknowledges the access to the
computational facilities at the University of North
Texas and Texas A&M University. The authors thank
Joseph Reibenspies for collecting the crystallographic
data and solving the structure.
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