First true square-planar Hg(II) compound: Synthesis and full characterization of trans-[Hg{Ph2PNP(O)Ph2-P,O}2]

Article abstract of DOI:10.1016/S1387-7003(02)00684-6

The reaction of Hg(O₃SCF₃)₂ with bis(diphenylphosphino)amine, Ph₂PNHPPh₂ (dppam), produces the novel, rare face-to-face complex [Hg₂(O₃SCF₃)₄(dppam)₂] (1). Treatment of 1 with Na₂N₂O₃ leads to trans-[Hg{Ph₂PNP(O)Ph₂P,O}₂] (2) via regioselective oxidation and simultaneous deprotonation of dppam. 2 is the first true square-planar Hg(II) compound. In 2 the coordination plane and the five-membered rings of the HgNOP₂ moieties are completely coplanar indicating strong π-bonding interactions. Both 1 and 2 have been fully characterized by X-ray structure analyses, NMR spectroscopy (¹⁹⁹Hg{¹H}, ³¹p{¹H}, ¹³C{¹H},¹H), ESI and FAB mass spectrometry, IR spectroscopy, elemental analyses, and melting points. Since in Hg(II) compounds relativistic effects favour linear coordination and Hg(II) donor atom preferences play a significant role, the Hg-P bond length of 2.4042(7) A? in 2 is short, whereas the Hg-O bond length of 2.7138(15) A? is long. In view of the X-ray structures of 1 and 2, it is shown that the delocalized charge in [Ph₂PNP(O)Ph₂]^- is responsible for the achievement of the square-planar coordination in a Hg(II) compound. A similar π-bonding effect has been observed in several square-planar complexes of Pt(II), Pd(II), and Ni(II) containing cis-1,2-bis(diphenylphosphino)ethene (cis-dppen).

Full text of DOI:10.1016/S1387-7003(02)00684-6

Inorganic Chemistry Communications 6 (2003) 61–67
www.elsevier.com/locate/inoche
First true square-planar Hg(II) compound: synthesis and
full characterization of trans-½HgfPh2PNPðOÞPh2-P; Og2Š
Rainer Haid , Rene Gutmann , Georg Czermak , Christoph Langes ,
a
a
a
a
b
a
c
Werner Oberhauser , Holger Kopacka , Karl-Hans Ongania , Peter Bruggeller
a,
*
€
a
Institut f€ur Allgemeine, Anorganische und Theoretische Chemie, Universit€at Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
Istituto di Chimica dei Composti Organometallici, (ICCOM)-CNR, Via J. Nardi 39, 50132 Firenze, Italy
b
c
Institut f€ur Organische Chemie, Universit€at Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
Received 22 July 2002; accepted 11 October 2002
Abstract
The reaction of HgðO3SCF3Þ2 with bis(diphenylphosphino)amine, Ph2PNHPPh2 (dppam), produces the novel, rare face-to-face
complex ½Hg2ðO3SCF3Þ4ðdppamÞ2Š (1). Treatment of 1 with Na2N2O3 leads to trans-½HgfPh2PNPðOÞPh2-P; Og2Š (2) via regiose-
lective oxidation and simultaneous deprotonation of dppam. 2 is the first true square-planar Hg(II) compound. In 2 the coordi-
nation plane and the five-membered rings of the HgNOP2 moieties are completely coplanar indicating strong p-bonding
interactions. Both
1
1
and
1
2
have been fully characterized by X-ray structure analyses, NMRspectroscopy
1
ð
199Hgf Hg; 31Pf Hg; 13Cf Hg; 1HÞ, ESI and FAB mass spectrometry, IRspectroscopy, elemental analyses, and melting points.
Since in Hg(II) compounds relativistic effects favour linear coordination and Hg(II) donor atom preferences play a significant role,
ꢀ
the Hg–P bond length of 2.4042(7) A in 2 is short, whereas the Hg–O bond length of 2.7138(15) A is long. In view of the X-ray
ꢀ
À
structures of 1 and 2, it is shown that the delocalized charge in ½Ph2PNPðOÞPh2Š is responsible for the achievement of the square-
planar coordination in a Hg(II) compound. A similar p-bonding effect has been observed in several square-planar complexes of
Pt(II), Pd(II), and Ni(II) containing cis-1,2-bis(diphenylphosphino)ethene (cis-dppen).
Ó 2002 Elsevier Science B.V. All rights reserved.
Keywords: Crystal structures; Mercury complexes; Square-planar mercury; Face-to-face complexes; Diphosphine complexes; Heterodifunctional
ligands
ꢀ
1. Introduction
distances of 2.707(7) and 2.90(2) A are indicative of a
five- or six-coordinate species. By contrast, linear two-
coordination is more predominant in the chemistry of
mercury than for any other element, where relativistic
effects strongly reduce intermolecular interactions
between HgX2 (X ¼ F, Cl, Br, I, H) molecules [3]. In
the case of phosphines the thoroughly investigated
complexes containing the core HgL2P2 (P, triorgano-
phosphine; L, anionic ligand) prefer tetrahedral coor-
dination [4]. Also phosphorus podal ligands increase
the coordination number of Hg(II) and force a tetra-
hedral geometry around the mercury ion [5]. The same
preference for Td is found in the virtual symmetry of
the first structurally characterized homoleptic phos-
The tolerance for many different coordination num-
bers and coordination geometries is typical of the co-
ordination chemistry of Hg(II) [1]. However, to the best
of our knowledge including a careful search of the
Cambridge Structural Database, no true square-planar
Hg(II) compound is known. ½HgðBMPAÞNCCH3Š
ðClO4Þ2, where BMPA is bis[(2-pyridyl)methyl]amine,
could be regarded as distorted square-planar complex,
but the four nitrogen atoms are displaced as much
ꢀ
as 0.5 A from their mean plane [2]. Furthermore,
close association to two axial perchlorates with Hg–O
2þ
phine complex of Hg(II), ½HgðPMe2PhÞ4Š [6]. Bulky
* Corresponding author. Tel.: +43-512-507-5115; fax: +43-512-507-
2934.
E-mail address: peter.brueggeller@uibk.ac.at (P. Bruggeller).
phosphides reduce the coordination number of Hg(II)
again and the central Hg atoms have a two-coordinate
¨
1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S1387-7003(02)00684-6

62
R. Haid et al. / Inorganic Chemistry Communications 6 (2003) 61–67
linear geometry in ½Hgðt-Bu2PÞ2Š and fHg½PðSiMe3Þ2Š2g2
[7,8].
(55.4%); m.p. ¼ 105 to 108 °C. IR(KBr, cm À1) m(N–H):
3170. ESI mass spectrum (MeOH): m/z 1385.1
Only recently it has been shown that bis(diph-
enylphosphino)amine, Ph2PNHPPh2 (dppam), favours
face-to-face complexes in the case of Hg(II) [9] and the
first fully characterized Hg(II) compound containing
dppam, ½Hg2ðHN2O3Þ2ðNO3Þ2ðdppamÞ2Š, has been pre-
sented. In this work the second fully characterized
complex of Hg(II) and dppam, ½Hg2ðO3SCF3Þ4
ðdppamÞ2Š (1), is given. 1 reacts with Na2N2O3 to pro-
duce the first true square-planar Hg(II) compound,
trans-½HgfPh2PNPðOÞPh2-P; Og2Š (2), where 2 is also
fully characterized. Although trioxodinitrate was first
described as a component of AngeliÕs salt, Na2N2O3, in
1896 [10], it remains the least familiar of the isolable
oxynitrogen anions [11–13] and the regioselective oxi-
dation and simultaneous deprotonation of dppam
leading from 1 to 2 via extrusion of mercury are unique.
The coordination chemistry of phosphorus containing
ligands with inorganic backbones as in 1 and 2 has been
the topic of much interest [14–18]. In 2 p-bonding
caused by theÀ p-delocalized nature of chelating
½Ph2PNPðOÞPh2Š leads to complete coplanar five-
membered rings of the HgNOP2 moieties and hence to
the first square-planar Hg(II) coordination. It is well
known, that the unsaturated backbone of the chelating
diphosphine cis-1,2-bis(diphenylphosphino)ethene (cis-
dppen) produces a comparable effect [19–24].
(1 – 3CF3SO3 þ 2MeOHÞþ, 1204.0 (1 – 4CF3SO3 þ
MeOHÞþ. 199Hgf Hg NMRðCH2Cl2Þ:
d 1159.4 (t,
1
1JðHg; PÞ ¼ 3528 Hz). 31Pf Hg NMR ðCH2Cl2Þ: d 71.2
1
2
1
(s). 13Cf Hg NMR(CH Cl2–d ): d 129.35–132.35 (m,
2
2
1
Ph). H NMR(CH Cl2–d ): d 7.25–7.97 (m, Ph). Anal.
2
Calcd for C52H42F12Hg2N2O12P4S4: C, 35.3; H, 2.39; N,
1.58; S, 7.25. Found: C, 35.1; H, 2.48; N, 1.54; S, 7.10.
Single crystals suitable for an X-ray structure analysis
with the composition ½Hg2ðO3SCF3Þ4ðdppamÞ2Š were
obtained from a CH2Cl2 solution of 1 at ambient tem-
perature.
2.2. Synthesis of trans-[Hg{Ph2PNP(O)Ph2-P,O}2] (2)
1 (0.113 mmol, 0.200 g) was dissolved in 12 ml of
CH2Cl2 and Na2N2O3 (0.452 mmol, 0.055 g) was added
with vigorous stirring and prevention of light. The re-
action mixture was stirred at ambient temperature for
several days. The slurry was filtered and the solvent was
evaporated completely. The residue was dissolved in
CH2Cl2 and a white powder was recrystallized from
CH2Cl2: yield 0.0401 g (35.4%); m.p. > 310 °C. IR
(KBr, cmÀ1) m(PNP): 1272, 1123, 1084. FAB mass
spectrum: m/z 1001.3 2þ. 31Pf Hg NMR(CH 2Cl2): d
1
69.8 (s, 1JðHg; PÞ ¼ 4076 Hz; Ph2P), 57.1 (s, 2JðHg; PÞ þ
3JðHg; PÞ ¼ 560 Hz; Ph2PO). Anal. Calcd for C48H40Hg
N2O2P4: C, 57.6; H, 4.03; N, 2.80. Found: C, 57.4; H,
4.15; N, 2.73. Single crystals suitable for an X-ray
structure analysis with the composition trans-
½HgfPh2PNPðOÞPh2-P; Og2Š were obtained by gas-phase
diffusion of n-hexane into a CH2Cl2 solution of 2 at
ambient temperature.
2. Experimental
Na2N2O3 was synthesized according to literature
methods [25] and dried in vacuo at 120 °C for several
hours. All other reagents were obtained commercially.
Dry solvents of purissimum grade quality were used for
the syntheses, NMRmeasurements, and crystallization
purposes. A Schlenk apparatus and oxygen-free, dry Ar
were used in the syntheses of 1 and 2. Solvents were
degassed by several freeze–pump–thaw cycles prior to
use. Positive chemical shifts are downfield from the
standards: aqueous HgðClO4Þ2 (2 mmol of HgO per cm3
2.3. Structure determination
Details of the crystals and data collections are sum-
marized in Table 1. In the case of 1, all data were col-
lected on a Siemens P4 diffractometer using omega-
scans. Cell refinement and data reduction were done by
the software of the Siemens P4 diffractometer [26], and
the empirical absorption corrections were based on w-
scans of nine reflections, respectively (v ¼ 78 to 102°,
360° scans in 10° steps in w) [27]. In the case of 2, the
data collection was performed on a Nonius Kappa CCD
diffractometer using combined /–x-scans. Cell refine-
ment, data reduction, and the empirical absorption
correction were done by Denzo and Scalepack programs
[28].
All structure determination calculations were carried
out using SHELXTL NT 5.10 including SHELXS-97
[29] and SHELXL-97 [30]. Final refinements on F 2 were
carried out with anisotropic thermal parameters for all
non-hydrogen atoms. In the case of 1, the N–H protons
were located and isotropically refined with fixed U. The
1
of 60% HClO4) for the 199Hgf Hg, 85% H3PO4 for the
1
1
31Pf Hg, and TMS for the 13Cf Hg and 1H resonances.
2.1. Synthesis of [Hg2(O3SCF3)4 (dppam)2] (1)
Dppam (0.195 mmol, 0.075 g) was dissolved in 8 ml
of CH2Cl2 and HgðO3SCF3Þ2 (0.195 mmol, 0.097 g) was
added with vigorous stirring. The reaction mixture was
stirred at ambient temperature for 3 days. A yellowish
solution and a white precipitate were obtained. Forty ml
of n-hexane was added and the slurry was stirred for 12
h. The slurry was filtered, and the white residue was
washed with toluene and dried in vacuo. A white pow-
der was recrystallized from CH2Cl2: yield 0.0955 g

R. Haid et al. / Inorganic Chemistry Communications 6 (2003) 61–67
63
Table 1
Structure determination data for ½Hg2ðO3SCF3Þ4ðdppamÞ2Š (1) and trans-½HgfPh2PNPðOÞPh2-P; Og2Š (2)
1
2
Formula
C52H42F12Hg2N2O12P4S4
1768.21
C48H40HgN2O2P4
1001.33
Formula weight
Colour; habit
Crystal system
Space group
Colourless; prismatic
Monoclinic
P21=n
13.420(4)
16.302(3)
14.071(3)
Colourless; prismatic
Triclinic
ꢁ
P1
ꢀ
a (A)
8.2073(1)
11.0828(2)
11.6938(2)
87.437(1)
81.638(1)
86.887(1)
1050.08(3)
213
ꢀ
b (A)
ꢀ
c (A)
a (°)
b (°)
c (°)
99.43(1)
ꢀ3
V (A )
3036.8(12)
178
T (K)
Z
Crystal dimensions (mm)
2
1
0:70 ꢁ 0:70 ꢁ 0:20
1.933
5.393
0:40 ꢁ 0:30 ꢁ 0:20
1.584
7.497
Dcalc ðMg mÀ3
Þ
l (mmÀ1
)
Diffractometer (Mo-Ka radiation)
2h Range (°)
Siemens P4
4.6–55.0
Nonius Kappa CCD
3.5–55.0
No. observed (I > 3rðIÞ (1), > 2rðIÞ (2)) reflections
3958
400
4818
340
No. parameters refined
R1 (on F)
0.0331
0.0790
0.978
0.0152
0.0385
1.06
wR2 (on F 2)
GOF
Largest difference peak (e A
ꢀÀ3
)
1.81
0.026
0.278–1.000
0.37
0.021
0.882–1.000
Rint
Transmission
aromatic hydrogen atoms were included using a riding
model with isotropic U values depending on Ueq of the
adjacent carbon atoms. For 2 all protons were located
and isotropically refined. This means that due to the low
R-values for both X-ray structures, the deprotonation at
nitrogen during the reaction leading from 1 to 2 can be
directly seen in their X-ray structures. Parallel refine-
ments with all hydrogen atoms in idealized positions
show no beneficial effects on the R-values or statistical
significance of both structures. In 1 the largest difference
1 þ 2N2O2À ! ½Hg2ðO3SCF3Þ ðPh2PNPPh2Þ Š
3
2
2
þ 2HN2OÀ þ 2O3SCFÀ
ð1Þ
ð2Þ
3
3
HN2OÀ ! HNO þ NOÀ
3
2
½Hg2ðO3SCF3Þ ðPh2PNPPh2Þ Š þ 4HNO
2
2
! 2 þ 2N2 þ 2H2O þ HgðO3SCF3Þ
ð3Þ
2
Also the decomposition reaction of HN2OÀ3 (Eq. (2))
leading to nitroxyl (HNO) and nitrite is well-known [13].
It has been shown, that the reduction of HNO (Eq. (3))
produces N2 [13] and the regioselective oxidation
ꢀÀ3
ꢀ
peak of 1.81 e A is located only 0.95 A away from Hg,
which can be typical for a 5d metal.
À
product ½Ph2PNPðOÞPh2Š forms 2 by five-membered
chelate ring closure and extrusion of HgðO3SCF3Þ .
2
3. Results and discussion
Since 1 belongs to the class of rare face-to-face
complexes of Hg(II) containing dppam [9] and in order
to confirm the above reaction sequence, the solid state
structure of 1 was determined by X-ray crystallography.
The crystal structure of 1 consists of two discrete
½Hg2ðO3SCF3Þ ðdppamÞ Š molecules per unit cell.
½Hg ðO3SCF3Þ4ðdppamÞ2Š is located on a centre of
Hg is unique among the transition metals in that it
readily forms two-, three-, and four-coordinate com-
pounds with the same ligand type [31]. Furthermore,
regioselective oxidation and simultaneous deprotona-
tion of dppam completely changes the coordination
behaviour of this ligand type with respect to Hg(II). This
reaction sequence leading from 1 to 2 is achieved by the
unique properties of the dibasic anion N2O23À, which is
destabilized upon protonation to the monobasic form
HN2OÀ3 [12]. The deprotonation of 1 (Eq. (1)) initiates
the following reaction sequence:
2
4
2
symmetry. Views of 1 are given in Fig. 1. Table 2 con-
tains selected bond distances and bond angles.
Fig. 1 clearly reveals the face-to-face coordination of
ꢀ
dppam in 1. The Hg ꢀ ꢀ ꢀ Hg distance of 3.2805(7) A in 1
is comparable to the corresponding parameter of
ꢀ
3.2213(9) A in ½Hg2ðHN2O3Þ2ðNO3Þ2ðdppamÞ2Š [9] and

64
R. Haid et al. / Inorganic Chemistry Communications 6 (2003) 61–67
Fig. 1. (a) View of ½Hg2ðO3SCF3Þ4ðdppamÞ2Š (1), showing the atomic numbering. (b) View of ½Hg2ðO3SCF3Þ4ðdppamÞ2Š (1), with the plane through
Hg(1), P(1), Hg(1A) and P(1A) perpendicular to the projection plane. The O3SCFÀ3 groups have been omitted for clarity.
indicates that there is no bonding interaction between
Table 2
ꢀ
the Hg(II) centres. The Hg–P bond lengths of
Selected bond distances (A) and bond angles (°) for ½Hg2
2.4213(13) and 2.4257(13) A in 1 are located within the
ꢀ
ðO3SCF3Þ4ðdppamÞ2Š (1) and trans-½HgfPh2PNPðOÞPh2-P; Og2Š (2)
ꢀ
typical range of 2.36–2.91 A [32–35]. This wide range of
values confirms the easy deformability of the coordi-
nation sphere of a d10 metal ion [36] and is reflected in
the Hg–O bond lengths of 1. The Hg(1)–O(1) and
Compound 1
Hg(1)–P(1)
Hg(1)–P(2)
Hg(1)–O(1)
Hg(1)–O(2)
O(2)–Hg(1)a
Hg(1)–O(6)
Hg(1)–Hg(1)a
P(1)–N(1)a
P(2)–N(1)
2.4213(13)
2.4257(13)
2.746(4)
2.893(5)
2.814(4)
2.679(4)
3.2805(7)
1.679(4)
1.681(5)
ꢀ
Hg(1)–O(6) distances of 2.746(4) and 2.679(4) A, re-
spectively, are relatively short and the Hg–O distances
of the bridging oxygen O(2) (see Fig. 1(a)) of 2.893(5)
ꢀ
and 2.814(4) A are relatively long indicating strong and
weak contacts of the O3SCFÀ3 groups to mercury.
Comparable Hg–O bond lengths of 2.526(5), 2.621(5),
P(1)–Hg(1)–P(2)
P(1)–Hg(1)–O(1)
P(2)–Hg(1)–O(1)
P(1)–Hg(1)–O(2)
P(2)–Hg(1)–O(2)
P(1)–Hg(1)–O(6)
P(2)–Hg(1)–O(6)
O(1)–Hg(1)–O(2)
O(1)–Hg(1)–O(6)
O(2)–Hg(1)–O(6)
N(1)a–P(1)–Hg(1)
N(1)–P(2)–Hg(1)
P(1)a–N(1)–P(2)
Hg(1)a–O(2)–Hg(1)
169.78(5)
94.35(11)
95.49(11)
86.35(10)
97.85(10)
91.08(10)
92.58(10)
50.13(12)
84.71(14)
134.27(13)
112.75(16)
113.30(16)
134.0(3)
ꢀ
and 2.864(5) A have been found in ½Hg2ðHN2O3Þ2
ðNO3Þ2ðdppamÞ2Š [9] and for the bidentate coordina-
tion of the nitrate groups in ½HgðNO3Þ2ðPPh3Þ2Š [35]
with corresponding Hg–O distances of 2.507(4) and
ꢀ
2.790(4) A. In line with these results, the range of
the Hg–O bond lengths in ½Hg2ðl-dpmaÞ2ðO3SCF3Þ2
ꢀ
ðCH3OHÞ3ŠðPF6Þ2 is 2.46–2.79 A, where dpma is
bis((diphenylphosphino)methyl)phenylarsine [37]. Fur-
thermore, a particularly strong interaction between the
central mercury atom and the oxygen atom of a
70.15(10)
ꢀ
methoxy group of 2.672(4) A is present in [HgI2
Compound 2
Hg(1)–P(1)
Hg(1)–O(1)
P(1)–N(1)
P(2)–N(1)
P(2)–O(1)
ðDMPPÞ], where DMPP is tris(2,6-dimethoxyphe-
nyl)phosphine [38]. Since all these Hg–O contacts are
within the sum of the van der Waals radii for the in-
2.4042(7)
2.7138(15)
1.5963(15)
1.6152(16)
1.5035(15)
ꢀ
teracting atoms of 3.1 A, the geometry around the
mercury ion in 1 should be regarded as six-coordinate
and distorted octahedral (Fig. 1(a)). In 1 the N–H
protons of dppam have been found in its X-ray struc-
ture and in the IRspectrum they are responsible for
m(N–H) at 3170 cmÀ1 in the typical range [16]. The
eight-membered ring of 1 is folded resulting in a flat
chair conformation, where the dihedral angle between
the planes through Hg(1), P(1), Hg(1A), P(1A), and
P(1A), N(1), P(2), respectively, is 18.8° (see Fig. 1(b)).
P(1)–Hg(1)–O(1)
P(1)a–Hg(1)–P(1)
P(1)a–Hg(1)–O(1)
N(1)–P(1)–Hg(1)
O(1)–P(2)–N(1)
P(2)–O(1)–Hg(1)
P(1)–N(1)–P(2)
76.83(4)
180.00(2)
103.17(4)
112.37(6)
117.42(8)
109.63(7)
123.35(10)
a Symmetry transformations used to generate equivalent atoms:
Àx; Ày; Àz.

R. Haid et al. / Inorganic Chemistry Communications 6 (2003) 61–67
65
In order to confirm the regioselective oxidation and
deprotonation of dppam during the reaction sequence of
Eqs. (1)–(3) and to characterize 2 definitely, also the
solid state structure of 2 was determined by X-ray
crystallography. The crystal structure of 2 consists of
one discrete trans-½HgfPh2PNPðOÞPh2-P; Og2Š molecule
per unit cell. Trans-½HgfPh2PNPðOÞPh2-P; Og2Š is lo-
cated on a centre of symmetry. Views of 2 are given in
Fig. 2. Table 2 contains selected bond distances and
bond angles.
Fig. 2 clearly reveals the first true square-planar co-
ordination of a Hg(II) compound. The Hg–P bond
ꢀ
length of 2.4042(7) A in 2 is significantly shorter than
ꢀ
the Hg–P distances of 2.4213(13) and 2.4257(13) A in 1,
ꢀ
but again located within the typical range of 2.36–2.91 A
The observed coplanarity of the coordination plane
with the backbone atoms in 2 is indicative of p-bonding
effects [19–24]. It is well-known, that short M–P bonds
as in 2 compared with 1 result from an enhancement of
M-to-P p-bonding caused by the unsaturated nature of a
chelating ligand [19]. This means, that in 2 a degree of
delocalization of the negative charge formally attached
to the deprotonated nitrogen atoms over the two five-
membered spiro-rings is present. In [PtHðPPh3Þ
ðNPP–HÞ], where NPP–H is deprotonated 2-pyridyl-
bis(diphenylphosphino)methane, a comparable delocal-
ization of the negative charge formally attached to the
deprotonated methine carbon over the four-membered
ring and some degree of p-interaction between the pyr-
idyl group and this delocalized charge have been found
[39]. Furthermore, there is evidence for p-delocalization
within the E–P–N–P–O and P–N–P–O backbones of the
E-ligating or O, E-chelating ligand ½Ph2PðOÞNPðEÞ
ꢀ
[32–35]. The Hg–O bond length of 2.7138(15) A in 2 is
comparable to the relatively short and strong Hg–O
contacts of the O3SCFÀ3 groups to mercury in 1.
Therefore, the coordination number of Hg(II) in 2
clearly is four (Fig. 2(a)). Furthermore, Fig. 2(b) shows
the nearly complete planarity of the five-membered
HgNOP2 rings. Due to crystallographic constraints
produced by the centre of symmetry this means, that
both five-membered chelate rings of 2 are also coplanar.
The largest deviation from a least-squares plane through
À
Ph2Š (E ¼ S or Se) [14]. In the case of ½Ph2PNPðOÞ
Ph2ŠÀ, short P–N bond lengths as in ½PdfPh2PNPðOÞ
ꢀ
Ph2-Pg2ðenފ ranging from 1.593(2) to 1.604(2) A [14]
indicate some degree of delocalization within the P–N–P
backbone. This effect is clearly visible in the significant
reduction of the P–N distances of 1.679(4) and 1.681(5)
ꢀ
ꢀ
A in 1 compared with 1.5963(15) and 1.6152(16) A in 2.
ꢀ
ꢀ
By contrast, the P–O bond length of 1.5035(15) A for
Hg(1), P(1), N(1), P(2), and O(1) is only 0.045 A for
N(1). Of course, the coordination plane itself consisting
of the atoms Hg(1), P(1), O(1), P(1A), and O(1A) is
completely planar again due to crystallographic con-
straints.
the coordinated phosphinoyl moieties in 2 is signifi-
cantly longer than the corresponding parameters of
ꢀ
1.465(5) and 1.478(4) A for P-unidentate Ph2PNHP
ðOÞPh2 in trans-½PtðCH3ÞClfPh2PNHPðOÞPh2-Pg2Š [16].
Fig. 2. (a) View of trans-½HgfPh2PNPðOÞPh2-P; Og2Š (2), showing the atomic numbering. (b) View of trans-½HgfPh2PNPðOÞPh2-P; Og2Š (2), with the
least-squares plane through Hg(1), P(1), N(1), P(2) and O(1) perpendicular to the projection plane.

66
R. Haid et al. / Inorganic Chemistry Communications 6 (2003) 61–67
Thus, in agreement with previous results [14–18,40] the
deprotonation and chelation of Ph2PNHPðOÞPh2
lengthens the P–O bond length and electronic delocal-
ization shortens the P–N bond length. In this context a
comparison of Figs. 1(b) and 2(b) is particularly helpful
demonstrating the onset of this electronic delocalization,
since the eight-membered ring of 1 is folded and the two
five-membered rings of 2 are planar.
Furthermore, Fig. 2(a) clearly revealÀs the mutual
trans arrangement of the ½Ph2PNPðOÞPh2Š ligands in 2.
This is in contrast to cis-½PtfPh2PNPðOÞPh2-P; Og2Š
[16], where the beneficial combination of an atom with
high trans influence like phosphorus and an atom with
low trans influence like oxygen in mutual trans position
dominates. Obviously, square-planar Hg(II) prefers a
symbiotic trans arrangement of identical atoms, which is
common in octahedral complexes containing compara-
ble heterodifunctional ligands like trans-½NiCl2ðPh2PðOÞ
CH2pyÞ2Š with trans arrangements O–Ni–O and N–Ni–
N [39]. Also the small P(1)–Hg(1)–O(1) chelate angle in
2 of only 76.83(4)° is unique, where the corresponding
parameters in cis-½PtfPh2PNPðOÞPh2-P; Og2Š are
86.1(2)° and 86.0(2)° [16].
where in ½Hgf2ÀC6H4PðOÞPh2g2Š the corresponding
31
dð PÞ value for the free P@O groups is 31.7 [41]. By
31
contrast, dð PÞ for the coordinated P@O groups in cis-
½PtfPh2PNPðOÞPh2 À P; Og2Š is 62.3 [16]. Thus, com-
pelling evidence for formation of 1 and 2 in solution is
provided by a combination of symmetry, shielding, and
heteronuclear coupling arguments. This means, that as a
first approximation the forces on 1 and 2 in solution are
of the same order as those in the solid [42].
It seems likely, that the formation of the two very
stable five-membered rings in 2 is the thermodynamic
driving force leading from 1 to 2. Therefore, a com-
parison with the well-known p-bonding effect in square-
planar complexes containing cis-dppen is interesting
À
[43]. ½Ph2PNPðOÞPh2Š belongs to the class of valuable
hemilabile ligands that have proven usefulness in several
important applications [45,46]. In this work the hetero-
difunctional properties of these ligands have lead to the
achievement of a unique square-planar coordination of
Hg(II).
Supplementary material
The uncertainties introduced by motional averaging
in solution and structural perturbations imposed by
packing constraints in the solid require that correlations
between solid-state structures and solution-state NMR
properties always be made cautiously [2]. In the case of
An X-ray crystallographic file, in CIF format, for
½Hg2ðO3SCF3Þ4ðdppamÞ2Š (1), trans-½HgfPh2PNPðOÞ
Ph2-P; Og2Š (2), and ½PdCl2 (cis-dppen)] (3) is available
from the authors on request. Further details on the
structure determination are also obtainable at the
Cambridge Structural Database.
1
1, the occurrence of the 199Hgf Hg NMRsignal as a
1
triplet and the single 31Pf Hg NMRpeak are in accor-
1
dance with its solid state structure. The JðHg; PÞ value
of 3528 Hz in 1 is comparable to the same parameter of
4430 Hz in ½Hg2ðHN2O3Þ2ðNO3Þ2ðdppamÞ2Š [9] and lo-
cated within the range of 1417–6300 Hz found for
pseudotetrahedral ½HgðPR3ÞnX4ÀnŠ complexes, where X
is an anionic ligand [4,36]. Also in the case of 2, the
Acknowledgements
€
We thank the Fonds zur Forderung der wissens-
chaftlichen Forschung, Austria, for financial support.
1
single 31Pf Hg NMRresonances at 69.8 and 57.1 cor-
respond to its solid state structure. The downfield signal
is attributed to the Ph2P groups, since its 1JðHg; PÞ
coupling of 4076 Hz is again in the typical range [4,9,36].
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