Thallium Carbodiimide
1
76(3)°) but, taking into account the limited experimental
mid. Silver cyanamide, on the other side, can also be con-
resolution, it may also be safely considered a carbodiimide
sidered an Ag NCN layer structure, but there are two per-
2
anion with two CϭN double bonds. In accord with the
pendicular layers with different coordination motifs
(AgϪNϪAgϪN and AgϪNCNϪAgϪNCN) resulting in
zig-zag chains [18]. Finally, the only reported main-group
III carbodiimide is a mixed-valent indium(I,III) carbodiim-
crystal-structure result, the infrared spectra of Tl NCN
2
exhibit only strong carbodiimide-type asymmetrical vi-
Ϫ1
brations, ν (NCN) ϭ 1851 cm , plus a strong defor-
as
Ϫ1
mation vibration, δ(NCN) ϭ 632 cm . In contrast, no
symmetrical νs band (around 1200 cm ) is observed
ide, In2.24(NCN) , obtained by a solid-state reaction [19],
3
Ϫ1
and it does not show strong structural similarities with
because such a breathing mode is IR-forbidden for the
Tl NCN.
2
2
Ϫ
[
NϭCϭN] carbodiimide unit although it would be al-
It has been argued [20] that the carbodiimide (or cyana-
mide) anion may be considered a pseudo-chalkogenide and
should therefore show structural similarities with the sulfide
anion. Indeed, one may calculate the molar volumes (or
volume increments) of the two species NCN and S on
the basis of the tabulated volume increments of the mono-
2Ϫ
lowed for the less symmetrical cyanamide [NϪCϵN]
anion. Thus, there is both structure-analytical as well as
vibrational evidence to phrase Tl NCN as a thallium car-
2
ϩ
2Ϫ
2Ϫ
bodiimide, despite the low absolute hardness of Tl (η ϭ
7.16 eV [7]) which would make it a cyanamide candidate
ϩ
according to the cyanamide/carbodiimide shape rule [8].
valent cations M [21] taking the crystal volumes of the
ϩ
The coordinations of the carbodiimide units by Tl ions
carbodiimides/sulfides of Li, Na, K, Ag and Tl as a refer-
ence; the numerical results are given in Table 5 and sketched
in Figure 3. On average, the carbodiimide anion is only
correspond to irregular [NCN]Tl octahedra. Each nitrogen
6
atom bonds to three thallium atoms and each thallium
atom is also bonded to three nitrogen atoms, but from one
side only (see Figure 2). Interestingly, the experimental
Ϫ
Table 5 Calculated molar volumes of NCN2 (top) and of S2Ϫ
anions (bottom) based on M
taining monovalent metal cations.
˚
TlϪN distances arrive at values between 2.52 and 2.72 A
2
NCN and M
2
S crystal structures con-
(
Table 4) such that the experimental TlϪN bond lengths
˚
are 0.19 to 0.43 A shorter than those that would have been
predicted on the basis of effective ionic radii (2.96 A be-
cause of r ϭ 1.46 A for N and 1.50 A for six-coordinate
Tl [9]). Clearly, these very short distances are a conse-
˚
ϩ
2Ϫ
2
M NCN
Cell volume; Z molar volume M volume [21] NCN volume
3Ϫ
˚
A
˚
3
cm /mol
3
cm /mol
3
cm /mol
3
˚
eff
ϩ
2
Li NCN [15] 117.8; 2
35.5
39.5
54.4
43.2
60.5
1.5
6.5
16
9
18.5
32.5
26.5
22.4
25.2
23.5
ϩ
quence of the small coordination number (3) of Tl , and
Na NCN [16] 131.2; 2
2
K
2
NCN [17] 180.6; 2
Ag NCN [18] 287.1; 4
Tl NCN 301.5; 3
(this work)
the calculation of an empirical bond-valence sum for the
2
covalent TlϪN bonds based on tabulated bond-valence
2
˚
parameters (r ϭ 2.29 A [10]) arrives at 1.42 for Tl1, 1.45
0
for Tl2, and 1.19 for Tl3 in Tl NCN. We note that corre-
sponding covalent bonds between Tl and N are quite rare
2
Cell volume; Z molar volume M volume [21] S2Ϫ volume
ϩ
2
M S
ϩ
˚
3
3
3
3
A
cm /mol
cm /mol
cm /mol
although the monovalent state is extremely common for in-
ϩ
Li S [22]
187.05; 4
279.73; 4
403.75; 4
226.83; 4
2325.7; 27
28.15
42.10
60.77
34.14
51.85
1.5
6.5
16
9
18.5
25.15
29.10
28.77
16.14
14.85
organic thallium compounds. For comparison, Tl ϪN
2
Na S [22]
2
˚
bonds are found in TlN (eight-fold coordination, 3.03 A)
3
K S [22]
2
[
2
11] and also in Tl Zn(N ) (ten-fold coordination,
.85Ϫ3.36 A) [12], but we consider these as more ionically
2
Ag S [23]
2
3 4
˚
2
Tl S [5]
bonded crystal structures. Indeed, organometallic molecules
involving monovalent thallium exhibit much shorter
ϩ
Tl ϪN bonds, for example when two-fold coordination
˚
(
2
2.42 and 2.47 A, [13]) and three-fold coordination (2 ϫ
˚
.60, 2.70 A, [14]) is encountered.
A structural comparison of all known (M ) NCN com-
ϩ
2
pounds is straightforward since such structural characteriz-
ations have been reported for all phases containing the alk-
ali metals (Li, Na, K) [15Ϫ17] and silver [18]. Because a
ϩ
˚
six-coordinate Tl ion has r ϭ 1.54 A which is not very
eff
ϩ
ϩ
ϩ
far away from those of K , Rb and Ag (1.44, 1.58 and
.27 A) [9], its chemistry might also resemble that of the
˚
1
former cations. The crystal structure of Tl NCN, however,
2
is an exception from the rule because it does not show any
obvious similarities with sodium/potassium carbodiimide
or silver cyanamide. The structures of the alkali carbodiim-
ides may be looked upon as arrangements of alternating
sheets of metal ions and NCN where the metal ion is
coordinated by five nitrogen atoms to give a square pyra-
2
Ϫ
Ϫ
Figure 3 Course of the molar volumes of NCN2 and S2Ϫ units
ϩ
in (M )
2
L phases as a function of the covalency.
Z. Anorg. Allg. Chem. 2007, 1339Ϫ1342
2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1341
Stork, Ludwig
