The Search for an Observable
Helium Complex
Adrian M. Gardner, Timothy G. Wright and Corey J. Evans
Metal Rare Gas Interactions
One could expect that the interactions between RG atoms and
metal cations would be purely physical.
The overall “strength” of the bond between the metal and RG is
approximately proportional to the polarizability of the RG.
Several studies have shown evidence of the formation of partial
covalent bonds between Xe and metals.
Gerry and co-workers1-3 have studied the RG–MX (RG = Ar, Kr,
Xe; M = Cu, Ag, Au; X = F, Cl, Br) using microwave spectroscopy.
1 Michaud, J.M., Gerry, M.C.L, J. Am. Chem. Soc., 2006, 128, 7613
2 Cooke, S.A., Gerry, M.C.L, J. Am. Chem. Soc., 2004, 126, 17000
3 Cooke, S.A., Gerry, M.C.L.., Phys. Chem. Chem. Phys., 2004, 13, 3248
But what about He and Ne?
In the 1980s Frenking and coworkers4-8 proposed a donoracceptor model which demonstrated He could form stable
chemical compounds with another species (X) providing that
there is;
•A suitable s- or σ-hole.
•A sufficiently high charge on X.
Several possible species have been proposed;8,9
He–CC2+
He–BCH
He–BeO
The coinage metal halide complexes appear to have these
requirements.
4. Koch, W. et al, J. Am. Chem. Soc., 1987, 109, 5917
5. Frenking, G. et al., 1988, 110, 8007
6. Frenking, G. et al., J. Phys. Chem. 1989, 93, 3397
7. Frenking, G., Cremer, D., Structure and Bonding, 1990, 73, 17
8. Frenking, G. et al., J. Am. Chem. Soc., 1990, 112, 4240
9. Koch, W. et al., Chem. Phys. Lett. 1986, 132, 330
RG-MX (M = Cu, Ag, Au; X= F, Cl; RG = He, Ne, Ar)
Geometry optimizations at the CCSD/aVDZ, CCSD/aVTZ and
CCSD/aVQZ levels of theory.
ECP10MDF, ECP28MDF and ECP60MDF with the aVXZ-PP
basis sets were used for Cu, Ag and Au respectively.
Energy calculations at the CCSD(T)/aVQZ and CCSD(T)/aV5Z.
Extrapolated the energy to the complete basis set limit.
All valence electrons of the RG’s, F and Cl as well as non-ECP
electrons of the metals were included in the correlation treatment.
10. Evans, C. J. et al., J. Phys. Chem. A., 2010, 114, 4446
Geometries and Dissociation Energies
Species
r(RG–M) / pm
r(M–F) / pm
Dissociation Energy (De)
CBS Limit / kJ mol-1
He–CuF
167.6
173.6
27.2
He–AgF
217.3
197.2
5.63
He–AuF
184.0
190.5
25.6
Ne–CuF
220.5
174.3
11.9
Ne–AgF
273.0
197.9
4.02
Ne–AuF
245.6
191.9
10.1
Ar–CuF
224.6
174.3
47.8
Ar–AgF
258.7
197.0
24.1
Ar–AuF
240.8
191.5
54.3
Geometries and Dissociation Energies
Species
r(RG–M) / pm
r(M–F) / pm
Dissociation Energy (De)
CBS Limit / kJ mol-1
He–CuF
167.6
173.6
27.2
He–AuF
184.0
190.5
25.6
Geometries and Dissociation Energies
Species
r(RG–M) / pm
r(M–F) / pm
Dissociation Energy (De)
CBS Limit / kJ mol-1
He–CuF
167.6
173.6
27.2
He–AgF
217.3
197.2
5.63
He–AuF
184.0
190.5
25.6
Geometries and Dissociation Energies
Species
r(RG–M) / pm
r(M–F) / pm
Dissociation Energy (De)
CBS Limit / kJ mol-1
He–CuF
167.6
173.6
27.2
He–AgF
217.3
197.2
5.63
He–AuF
184.0
190.5
25.6
Ne–CuF
220.5
174.3
11.9
Ne–AgF
273.0
197.9
4.02
Ne–AuF
245.6
191.9
10.1
Geometries and Dissociation Energies
Species
r(RG–M) / pm
r(M–F) / pm
Dissociation Energy (De)
CBS Limit / kJ mol-1
He–CuF
167.6
173.6
27.2
He–AgF
217.3
197.2
5.63
He–AuF
184.0
190.5
25.6
Ne–CuF
220.5
174.3
11.9
Ne–AgF
273.0
197.9
4.02
Ne–AuF
245.6
191.9
10.1
Ar–CuF
224.6
174.3
47.8
Ar–AgF
258.7
197.0
24.1
Ar–AuF
240.8
191.5
54.3
Analysis of Bonding within the RG-MF complexes
Natural Bond Order (NBO) analysis was carried out using the CCSD
density using the aVTZ basis set in G03.
He+CuF
HeCuF
He+AgF
HeAgF
He+AuF
HeAuF
He
0.00
0.06
0.00
0.02
0.00
0.07
M
0.83
0.74
0.85
0.81
0.68
0.60
F
-0.83
-0.80
-0.85
-0.83
-0.68
-0.67
Analysis of Bonding within the RG-MF complexes
The inductive interactions present between the RG and the MF can be
approximated by;11
Eind
Eind
   2

 2
6
4

0  4 0 r

2
   qeff

 
4
 40  80 r
11. Thomas, J. M. et al., J. Am. Chem. Soc., 2004, 126, 1235
Analysis of Bonding within the RG-MF complexes
Complex
μ/μind
qeff/μind
De
He–CuF
3
9
27
He–AgF
1
3
6
He–AuF
1
2
26
He–Cu+
-
10
11
He–Ag+
-
4
5
He–Au+
-
4
5
All values are in kJ mol-1
Analysis of Bonding within the RG-MF complexes
LUMO of AuF
The electronegativity of the F will pull electron density away from the
metal atom.
The HOMO-LUMO gap is smaller in the MF than in the M+.
Comparison of the bonding of He-MF and He-MCl
Species
r(RG–M) / pm
r(M–X) / pm
Dissociation Energy (De)
CBS Limit / kJ mol-1
He–CuF
167.6
173.6
27.2
He–CuCl
175.3
205.6
17.8
He–AgF
217.3
197.2
5.63
He–AgCl
226.2
228.3
4.52
He–AuF
184.0
190.5
25.6
He–AuCl
197.4
220.8
13.6
Conclusions
The dissociation energies of the He-MX are higher than the NeMX complexes.
The dissociation energy of the He-MF complexes are higher than
He-MCl complexes.
Is the bonding in these species covalent?
Potentially, the dissociation energies of the He-MX complexes are
sufficiently high that these species may be observed experimentally.