Gold is smaller than silver. Crystal structures of [bis(trimesitylphosphine)gold(I)] and [bis(trimesitylphosphine)silver(I)] tetrafluoroborate

Full text of DOI:10.1021/ja961363v

7006
J. Am. Chem. Soc. 1996, 118, 7006-7007
Table 1. Crystal Data for [(Mes3P)2M]BF4 with M ) Ag, Au
(Both at -68 °C)
Gold Is Smaller than Silver. Crystal Structures of
[Bis(trimesitylphosphine)gold(I)] and
-
-
formula
crystal system trigonal
[Mes3P-Au-PMes3]+BF4 [Mes3P-Ag-PMes3]+ BF4
[Bis(trimesitylphosphine)silver(I)] Tetrafluoroborate
trigonal
P3121
3
Angela Bayler,† Annette Schier,†
space group
Z
P3121
3
Graham A. Bowmaker,‡ and Hubert Schmidbaur*,†
a, b, Å
c, Å
15.942(1)
18.206(2)
4006.7(4)
15.900(2)
18.269(2)
3999.9(9)
Anorganisch-chemisches Institut der
Technischen UniVersita¨t Mu¨nchen
Lichtenbergstrasse 4, D-85747 Garching, Germany
Department of Chemistry
V, Å3
We have now discovered that the pair of title compounds
meets all criteria on which to base the desired direct comparison,
and from accurate single-crystal work we find that gold(I) is
indeed much smaller than silver(I), by almost 0.1 Å.12
The two reference complexes are readily prepared from
AgBF4 and two equiv of Mes3P in dichloromethane (95% yield,
mp 193 °C) or from equimolar quantities of (Mes3P)AuCl,
Mes3P, and AgBF4 in CH2Cl2 (98% yield, mp 232 °C),
respectively. The products can be obtained as large, transparent,
isomorphous crystals (trigonal, space group P3121, Z ) 3),
which are stable to air, moisture, and light at ambient temper-
ature.13 Their analytical and spectroscopic data are in full
agreement with the proposed compositions.14
Selected crystal data for [(Mes3P)2M]BF4 presented in Table
1 show the close crystallographic resemblance of the two unit
cells, which suggest a very similar crystal field environment
for the individual components. Both compounds are ionic in
the crystal with no significant sub-van der Waals contacts
between the ions. The cations have a crystallographically
imposed 2-fold axis passing through the metal atom perpen-
dicular to the P-M-P axis and relating the two phosphine
ligand propellers, which thus have the same directionality (left-
or right-handed propellers).14 The metal atoms are essentially
linearly two-coordinate with bond angles which deviate from
perfect collinearity by less than 0.3°. Selected distances and
angles are compared in Table 2.
The UniVersity of Auckland, PriVate Bag 92109
Auckland, New Zealand
ReceiVed April 25, 1996
Although it may have gone largely unnoticed, there is
considerable confusion in handbooks of physical data as well
as in chemistry textbooks and periodic tables concerning the
relative sizes of silver and gold atoms. Values quoted for the
ionic or covalent radii for the most common oxidation state +1
are either approximately equal for the two metals or larger for
gold than for silver.1-4 The Pauling covalent radii for the two
metals are essentially equal, which is due to the fact that the
“metallic radius” in the close-packed cubic lattices happens to
be virtually the same [the lattice constants are 4.0862 (Ag) and
4.07825 Å (Au),5 and the nearest-neighbor interatomic distances
are 2.889 (Ag-Ag) and 2.884 Å (Au-Au) for coordination
number 12].6
On the other hand, recent theoretical calculations including
relativistic and correlation effects consistently predict that gold
should be significantly smaller than silver, a phenomenon which
is generally referred to as the “relativistic contraction”.7-9 In
more qualitative terms, the concept of the “Lanthanide contrac-
tion”,7,8 employed successfully for other radius discontinuities
in the periodic table, also points in the same direction.10 It
appears, however, that no attempt has been made to settle this
simple question by an experiment which can give unambiguous
results.
The structure of the cation of the gold complex is shown in
Figure 1, and Figure 2 offers a superposition of the structures
of the gold and silver complexes. It is obvious from this
diagram that there is almost perfect agreement of all details
except for the M-P distance, which is smaller for M ) Au
than for M ) Ag by 0.09(1) Å. Assuming a covalent radius of
The most straightforward approach to this problem would
be a comparison of metal-to-ligand bond lengths in a set of
complexes involving (a) the same ligands and counterions, (b)
the same coordination number and geometry, (c) an isomorphous
crystal lattice, and (d) equal experimental conditions. More
often than not these conditions are not fulfilled, since Ag(I)
and Au(I) cations form compounds which differ significantly
in their basic structure,11,12 such that a direct comparison is not
meaningful.
(13) Crystal Structure Determination. The samples were mounted in
glass capillaries on an Enraf-Nonius CAD4 diffractometer and used for
measurements of precise cell constants and intensity data collection. During
data collection, three standard reflections were measured periodically as a
general check of crystal and instrument stability. No significant changes
were observed. Graphite-monochromated Mo KR radiation was used. Both
structures were solved by Patterson methods and refined by full matrix least-
† Technische Universita¨t Mu¨nchen.
squares calculations on F2. Crystal Data for C54H66P2AuBF4. Mr
)
‡ The University of Auckland.
1060.85, colorless crystals of dimensions 0.25 × 0.30 × 0.50 mm, trigonal,
a, b ) 15.942(1) Å, c ) 18.206(2) Å, space group P3121, Z ) 3, V )
4006.7(4) Å3, Fcalcd ) 1.319 g cm-3, F(000) ) 1620; T ) -68 °C. Data
were corrected for Lorentz, polarization, and absorption effects [µ(Mo KR)
) 28.47 cm-1]. 11 882 measured [(sin θ/λ)max ) 0.62 Å-1] reflections,
5682 independent reflections; 295 refined parameters, wR2 ) 0.0764, R )
0.0288 for 5645 reflections with Fo g 4σ(Fo). Residual electron densities:
+1.68/-0.76 eA-3 . Absolute structure parameter: -0.019(7). Crystal Data
for C54H66P2AgBF4. Mr ) 971.69, colorless crystals of dimensions 0.35
× 0.35 × 0.45 mm, trigonal, a, b ) 15.900(2) Å, c ) 18.269(2) Å, space
group P3121, Z ) 3, V ) 3999.8(9) Å3, Fcalc ) 1.210 g cm-3, F(000) )
1524; T ) -68 °C. Data were corrected for Lorentz and polarization but
not for absorption effects [µ(Mo KR) ) 4.84 cm-1]. 5815 measured [(sin
θ/λ)max ) 0.62 Å-1] reflections, 2914 independent reflections; 295 refined
(1) Emsley, J. The Elements, 2nd ed.; Clarendon: Oxford, 1991.
(2) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
Pergamon: Oxford, 1984; p 1368.
(3) Huheey, J.; Keiter, E.; Keiter, R. Inorganic Chemistry; Harper Collins
College: New York, 1993.
(4) (a) VCH Periodic Table of the Elements; VCH: Weinheim, 1980.
(b) CRC Handbook of Chemistry and Physics, 76th ed.; CRC Press: New
York, 1995-96. Based on: Shannon, R. D. Acta Crystallogr. 1976, A32,
751.
(5) Wyckhoff, R. G. Crystal Structures, 2nd ed.; Interscience Publish-
ers: New York, 1958; Vol. 1, p 10.
(6) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1980; p 403.
(7) Pyykko¨, P. Chem. ReV. 1988, 88, 563.
parameters, wR2 ) 0.0835, R ) 0.0320 for 2895 reflections with Fo g
(8) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker G. A.;
Boyd, P. D. W. J. Chem. Phys. 1989, 91, 1762.
4σ(Fo). Residual electron densities: +0.66/-0.31 eA-3. Absolute structure
parameter: -0.07(3). All hydrogen atoms in both structures were calculated
and allowed to ride on their corresponding C atoms with fixed isotropic
contributions [Uiso(fix) ) 0.08 Å2]; all non-H atoms were refined with
anisotropic displacement parameters. The BF4 counterions were disordered
on a 2-fold axis. Refinement of both structures in the enantiomorphic space
group P3221 led to significantly worse R and GOF values and to absolute
structure parameters of 1.0.
(9) Schwerdtfeger, P.; Boyd, P. D. W.; Burrell, A. K.; Robinson, W. T.;
Taylor, M. J. Inorg. Chem. 1990, 29, 3593.
(10) Liao, M. S.; Schwarz, W. H. E. Acta Crystallogr. 1994, B50, 9.
(11) Lancashire, R. J. ComprehensiVe Coordination Chemistry; Wilkin-
son, G., Ed.; Pergamon: Oxford, 1987; Vol. 5, p 775.
(12) (a) Puddephatt, R. J. ComprehensiVe Coordination Chemistry;
Wilkinson, G., Ed.; Pergamon: Oxford, 1987; Vol. 5, p 861. (b) Puddephatt,
R. J. The Chemistry of Gold; Elsevier: Amsterdam, 1978.
(14) Bayler, A.; Bowmaker, G. A.; Schmidbaur, H. Inorg. Chem., in
press.
S0002-7863(96)01363-7 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 29, 1996 7007
Table 2. Selected Bond Lengths (Å) and Angles (deg) for the
Title Compounds
and Table 2 reveals that the two ligands are in a strain-free
staggered conformation with deviations in their geometrical
parameters within the limits of experimental error. The positions
of the BF4 anions relative to the cations are also virtually
unchanged for M ) Ag and Au. Thus the internal and external
force fields in the lattice exerting influences on the P-M-P
units should be comparable.
parameter
M-P
P-C11
P-C21
P-C31
M ) Ag
M ) Au
2.4409(9)
1.827(4)
1.829(4)
1.833(4)
2.3525(10)
1.820(5)
1.829(5)
1.834(4)
It should be noted that Ag/Au-PR3 distances are found to
be smaller in most other complexes of these metals than in the
title compounds, depending on the size and electronic structure
of the substituents R.16,17
P-Au-P′
179.80(6)
107.22(13)
105.74(12)
107.50(12)
179.72(7)
107.5(2)
105.85(13)
107.82(13)
Au-P-C11
Au-P-C21
Au-P-C31
However, for small ligands there is generally also a growing
interference of the “third party” in the coordination sphere of
the metals (the counterions becoming ligands), such that the
structural characteristics become fundamentally different; e.g.,
[(Ph3P)2AuCl] features trigonal planar tricoordinate gold centers,
while the corresponding silver complex is dimeric, with tetra-
coordinate silver.18,19
C11-P-C21
C21-P-C31
C31-P-C11
113.8(2)
111.6(2)
111.6(2)
113.3(2)
111.5(2)
111.5(2)
Unfortunately, crystals of [(Mes3P)2Cu]BF4 obtained from
various solvents are not isomorphous with the Ag and Au
analogues and were always found to contain solvent. Notwith-
standing, the structure (of a CH2Cl2 solvate) was also solved
and gave an average Cu-P distance of 2.242(2) Å, placing
copper even smaller than gold with a radius of 1.13 Å. These
data refer to a different crystal environment and are therefore
not considered any further in this account, except to remark
that the shorter Cu-P distance supports the claim made above
that steric effects between the the two Mes3P ligands in the
silver and gold complexes are not important in determining the
M-P bond lengths.
The results presented here are in full agreement with data
obtained from theoretical calculations for the coinage metals,
which predict an increase of the atomic radii on going from
copper to silver, but a decrease on continuing from silver to
gold.7-10 Strictly speaking, the values are only valid for the
+1 oxidation state, but it is likely that gold should now be taken
as smaller than silver in most of its chemistry. For two-
coordinate M(I) compounds, radii of 1.13 (Cu), 1.33 (Ag), and
1.25 (Au) Å sould be tabulated.
Figure 1. Molecular structure of the cation of [(Mes3P)2Au]BF4
(ORTEP, 50% probability ellipsoids) with atomic numbering. The cation
has a crystallographic 2-fold axis passing through the Au atom.
Acknowledgment. This work was supported by DFG, FCI, and
the Alexander von Humboldt Foundation (G.A.B.) andsthrough the
donation of chemicalssby Degussa AG and Heraeus GmbH. The
authors are grateful to Mr. J. Riede for determining the X-ray data
sets.
Supporting Information Available: Tables of crystal data, bond
lengths, atomic coordinates, and thermal parameters for both compounds
(16 pages). See any current masthead page for ordering and Internet
access instructions. The same material has also been deposited at the
Fachinformationszentrum Karlsruhe, Gesellschaft fu¨r wissenschaftlich-
technische Information mbH, D-76344 Eggenstein-Leopoldshafen and
is available on request on quoting CSD 405279/405280.
Figure 2. Superposition of the structures of the cations of [(Mes3P)2M]-
BF4 with M ) Ag, Au, drawn with coinciding phosphorus positions in
the left part of the molecules.
tetracoordinate phosphorus as r(P)cov ) 1.11 Å,15 the covalent
radii of two-coordinate Ag and Au are estimated as 1.33 and
1.25 Å, respectively, a reduction in radius of as much as 6%
from Ag to Au.
JA961363V
(16) Baker, L.-J.; Bowmaker, G. A.; Camp, D.; Effendy; Healy, P. C.;
Schmidbaur, H.; Steigelmann, O.; White, A. H. Inorg.Chem. 1992, 31, 3656.
Baker, L.-J.; Bott, R. C.; Bowmaker, G. A.; Healy, P. C.; Skelton, B. W.;
Schwerdtfeger, P.; White, A. H. J. Chem. Soc., Dalton Trans. 1995, 1341.
(17) Berners-Price, S. J.; Colquhoun, L. A.; Healy, P. C.; Byriel, K. A.;
Hanna, J. V. J. Chem. Soc., Dalton Trans. 1992, 3357.
(18) Bowmaker, G. A.; Dyason, J. C.; Healy, L. A.; Engelhardt, L. M.;
Pakawatchai, C.; White, A. H. J. Chem. Soc., Dalton Trans. 1987, 1089.
(19) Bowmaker, G. A.; Effendy; Hanna, J. H.; Healy, P. C.; Skelton, B.
W.; White, A. H. J. Chem. Soc., Dalton Trans. 1993, 1387. Cassel, A.
Acta Crystallogr., Sect. B 1979, 35, 174.
Possible sources of error to be considered are steric interac-
tions of the two bulky Mes3P ligands. Inspection of Figure 1
(15) Corbridge, D. E. C. Phosphorus. An Outline of its Chemistry,
Biochemistry, and Technology; Elsevier: Amsterdam, 1978; pp 440-442.