2 5 6 2
[M(XeF ) ](PF )
+
temperature ultimately yielding only the corresponding binary
metal fluoride. Therefore, to obtain the appropriate stoichi-
ometry it is necessary to remove HF at -40 °C and to
maintain the solids at low temperature. In the cases of the
related metal hexafluoroarsenates or hexafluoroantimonates
the bridging XeF
both salts than any other ligands, it seems likely that the
2
F ligands are farther from the M2 ion in
2
+
charges on other F ligands close to M , provided both by
-
the partially ionized nonbridging XeF
2
and the PF
6
, exceed
-0.5 e.
-
anions to the M2+ is a
necessity of good lattice energy, but any loss of that energy
caused by the slight displacement by the partially ionized
with the metal-coordinated XeF
could not be used because formation of the rather stable and
aHF soluble compounds Xe AF or XeFAF (A ) As,
2
, this synthetic approach
The close approach of the PF
6
2
F
3
6
6
1
9,20
Sb)
resulted.
nonbridging XeF
energy derived from the higher charge of the more ionized
F ligands of the nonbridging XeF
In isostructural salts (e.g., those of smaller alkali metal
2
must be compensated for by the Coulomb
Crystal Structures. The unit cell volumes of these
stoichiometrically and structurally related salts indicate that
2
.
2
+
3
the effective volume of the Ca ion must be about 4 Å
2+
25
-
-
larger than that of the Cd ion. This is in accord with the
greater effective nuclear charge of the Cd2 ion, as illustrated
by the second ionization potentials of the atoms: Cd , 16.91
eV; Ca , 11.87 eV. Thus, the greater F-ligand coordination
cations ) the PF
6
anion is smaller than the AsF
6
anion.
+
This could be a factor in causing the change in stoichiometry
to [M(XeF ) ](AsF ) (M ) Ca , Cd ), when the latter
2 4 6 2
anion is substituted for the former. However, the fluoro-
arsenate anion is a significantly weaker F donor than the
fluorophosphate anion, the gaseous ionization energies for
5
f EF + F being 4.08 (E ) P) and
.42 eV (E ) As). This must be a consequence of greater
2+
2+
2+ 3,5
2+
21
2
+
-
of Cd is readily understood.
2
6
Initial considerations lead to the expectations that if these
F ligands all bore the same charge (the impact of their
pendant groups is ignored), the F ligands would all be on a
spherical surface, centered on the cation with the coordination
-
-
the processes EF
6
4
effective nuclear charge at the As center than at the P atom.
It is therefore to be expected that the F ligands of the AsF
-
2
+
2+
6
sphere of the Ca ion being larger than that of the Cd
-
will bear less charge than those in the PF
6
anion. The
6
salts, when compared with
salts from this study, support this view.
ion. But if the charges on the F ligands are not equal, a ion
distortion of the coordination shell is anticipated. Clearly,
the most negatively charged F ligands will approach the
cation more closely, and F‚‚‚F interactions with the other,
less charged F ligands will repel the latter so that they are
further from the cation. From the data in Table 2, taken
together with Figures 1 and 3, we note first that the closest
-
3,5
structural details of the AsF
-
those of the PF
6
-
The calcium salts provide two AsF
6
complexes for the
molecules/Ca2 and the other
+
comparison, one with 4 XeF
2
2
+
with 2.5 XeF
2
molecules/Ca .
+
In the [Ca(XeF
2
)
2.5](AsF
6
)
2
salt, the Ca2 coordination by
F ligands to the M2 are the four-rod shaped XeF
that are linked through only one F ligand to one M . In
accord with the higher effective nuclear charge on the Cd
ion, the F 2-5 ligands are closer to Cd2+ than to Ca , indeed
+
F ligands is rather like that in the [Ca(XeF
2
)
5
](PF salt,
6 2
)
2
molecules
2+
2
+
with eight F ligands at less than 2.6 Å from the Ca ion
and an additional F at about 2.8 Å. The Ca2 F-ligand
+
2
+
2+
coordination in the [Ca(XeF
Ca-F distances being less than or equal to 2.4 Å. In all of
these structures there are bridging XeF molecules coordi-
anions each coordinated
2 4 6 2
) ](AsF ) salt is eight with all
on average 0.07 Å closer. The F ligands of these nonbridging
2
XeF
2
molecules are being pulled away form their Xe atoms.
molecules is on the ionization pathway:
2
+
-
nated to two Ca , and two EF
6
Each of these XeF
F-Xe-F f F-Xe + F . The other F ligand approaches
the Xe atom more closely (becoming more XeF like) as
the F atom approaching the M2+ cation moves away from
2
2
+
+
- 22
to one Ca . The closely packed 8-fold Ca‚‚‚F coordination
-
+
sphere is somewhat more regular in the AsF
6
salt than in
-
the PF
6
salt (see ref 27). The striking difference, when
compared with the [Ca(XeF ) ](PF )
2 5 6 2
structure, concerns the
molecules (see ref 28).
the Xe atom. It should also be noted that the roughly
tetrahedral positioning of the four nonbridging XeF ligands
2
Ca-F distances of the bridging XeF
2
Overall, there is a closer approach of the bridging XeF
2
to
salt. This is
ligands being less strongly attracted
2
. This contrasts with the situation
tends to maximize the separation of the more positive XeF
ends of the partially ionized molecules. Clearly, the F ligand
2
+
-
-
the Ca in the AsF
consistent with the AsF
6
salt than in the PF
6
-
approaching the M2 must be more negatively charged than
+
6
2
+
to Ca , relative to XeF
in the more symmetrical, bridging XeF
the effect of crowding out the bridging XeF
molecular XeF , the effective F ligand negative charge has
been reported to be approximately at -0.5 e.2
2
molecules. This has
2
ligands. In
(
23) Jortner, J.; Rice, S. A.; Wilson, E. G. J. Chem. Phys. 1963, 38, 2302-
305.
(24) Jortner, J.; Wilson, E. G.; Rice, S. A. J. Am. Chem. Soc. 1963, 85,
2
2
3,24
Because
814-815.
(
25) Kemmitt, R. D. W.; Russell, D. R.; Sharp, D. W. A. J. Chem. Soc.
(
17) Zˇ emva, B. Noble Gases. In Encyclopedia of Inorganic Chemistry;
King, R. B., Ed.; John Wiley & Sons: New York, 1994; Vol. 5, pp
1963, 4408-4413.
(26) Krossing, I.; Raabe, I. Chem.sEur. J. 2004, 10, 5017-5030.
(27) In [Ca(XeF2)4](AsF6)2, Ca-F distances range from 2.27 to 2.40 Å,
and in [Ca(XeF2)2.5](AsF6)2, they range from 2.33 to 2.44 Å. In the
phosphorus salt the eight close Ca-F distances are 2.27, 2.32, 2.33
twice, 2.36, 2.40, 2.55, and 2.57 Å.
2660-2680.
(
(
(
18) Zalkin, A.; Ward, D. L.; Biagioni, R. N.; Templeton, D. H.; Bartlett,
N. Inorg. Chem. 1978, 17, 1318-1322.
19) Sladky, F. O.; Bulliner, P. A.; Bartlett, N. J. Chem. Soc. 1969, 2179-
2188.
(28) In [Ca(XeF2)5](PF6)2 (Figure 1, Table 2), CaF6 ) 2.40 and CaF1 )
2.55 Å, whereas, in [Ca(XeF2)4](AsF6)2, the Ca-F distances for the
four bridging XeF2 ligands are 2.27, 2.36 twice, and 2.39 Å. In [Ca-
(XeF2)2.5](AsF6)2 (in which all XeF2 groups are each bridging two
20) Frlec, B.; Holloway, J. H. J. Chem. Soc., Dalton Trans. 1975, 535-
540.
(
21) Pearson, R. G. Inorg. Chem. 1988, 27, 734-740.
2+
(22) Zˇ emva, B.; Jesih, A.; Templeton, D. H.; Zalkin, A.; Cheetham, A.
K.; Bartlett, N. J. Am. Chem. Soc. 1987, 109, 7420-7427.
Ca ) the bridging XeF2 Ca-F distances are 2.36, 2.42 twice, 2.44,
and 2.46 Å.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1041