Trans influence and covalent bonding in a new octahedral lanthanum(III) complex of diphenylmorpholinyl phosphinamide

Article abstract of DOI:10.1016/j.ica.2012.02.003

The ligand (C₆H₅)₂P(O)NC₄H ₈O (L) was synthesized and used to prepare new octahedral complex mer-[(C₆H₅)₂(NC₄H ₈O)PO]₃LaCl₃

Full text of DOI:10.1016/j.ica.2012.02.003

Inorganica Chimica Acta 386 (2012) 8–12
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Inorganica Chimica Acta
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Trans influence and covalent bonding in a new octahedral lanthanum(III) complex
of diphenylmorpholinyl phosphinamide
⇑
Khodayar Gholivand , Hamid R. Mahzouni
Department of Chemistry, Faculty of Basic Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The ligand (C6H5)2P(O)NC4H8O (L) was synthesized and used to prepare new octahedral complex mer-
[(C6H5)2(NC4H8O)P@O]3LaCl3 (LaL3Cl3). In the solid state structure of LaL3Cl3, the chloride anion trans
to neutral ligand L3 (Cl2) has an inverse trans influence, while the La–O3 (trans to Cl2) distance is slightly
shorter than the La–O1 and La–O2 distances (La–O1 trans to La–O2). On the contrary, the La–O3 distance
is considerably longer than the La–O1 and La–O2 distances in the gas phase structure of LaL3Cl3, as a
result of the normal trans influence of Cl2. The La–Cl2 (trans to La–O3) binding energy is larger in mag-
nitude than the calculated values for the La–Cl1 and La–Cl3 bonds. The directional dependence of La–
Received 20 November 2011
Received in revised form 25 January 2012
Accepted 2 February 2012
Available online 13 February 2012
Keywords:
Covalent bonding
DFT calculation
Lanthanide complex
Phosphoryl
Trans influence
X-ray crystallography
ligand bonding is in agreement with the trends in calculated electron density (q) values at bond critical
points (bcp’s) and wiberg bond indices. Based on the results of atoms in molecules (AIM) analysis, the
positive values for the electronic energy density, H(r), at the bcp of La–O bond paths suggest an ionic
character for the La–O bonds, and the small negative values for H(r), at the bcp of La–Cl bonds, indicate
a partial covalent character of the La–Cl interaction. The NBO analysis reveals a partial occupancy of La 5d
and 6s orbitals which is originated from the ligand to metal charge transfer (LMCT) in complex LaL3Cl3,
agreeing with the loss of electron density from the coordinated ligands L and chloride counterions.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
(EDA), we have recently shown that there are significant contribu-
tion of donative and back-donative interactions in Ln–ligand
The coordination chemistry of rare earth elements has become
the focus of experimental and theoretical researches because of its
implication in the nuclear waste management [1–5]. The highly
oxophilic nature of lanthanide (Ln) cations gives them a potential
to establish a strong non-covalent interaction with highly polar-
ized functional groups [6–8]. The ligands bound to Ln ions experi-
ence a spherical electrostatic potential, since the 4f electrons do
not participate in bonding [9]. Although Ln–ligand bonds are pre-
dominately electrostatic, some degree of covalency has been also
characterized and the concept of covalency is therefore an impor-
tant issue in Ln chemistry [10,11]. On the other hand, bis-arene
lanthanides have been described in term of significant contribution
bonding, while the covalent character of Ln–ligand interaction
can be up to ꢀ55% of total attractive forces. The ligand to metal
charge transfer (LMCT) in this compounds is increased when the
complexant molecule changes from phosphine oxide to phospho-
ramide, in line with increase in the strength of Ln–ligand bond
[14]. The synthesis and crystal structures of a great number of
lanthanide complexes of phosphine oxides have been reported
[15–18], but crystal structure of such complexes with monoden-
tate phosphoramides [19,20] have been limited to a few reports.
In this regard, we have previously reported the X-ray structure of
some La(III) complexes with phosphoric triamide ligands [21]. In
the present work, a phosphinamide ligand with the formula
(C6H5)2P(O)NC4H8O (L), representative of both phosphoramides
and phosphine oxides, was synthesized and used to prepare
octahedral complex mer-[(C6H5)2(NC4H8O)P@O]3LaCl3 (LaL3Cl3).
Schematic representation of the structures is illustrated in Fig. 1.
The crystal structure of LaL3Cl3 was determined by X-ray crystal-
lography. The geometry and electronic structure of LaL3Cl3 have
been also investigated at DFT level to explain the concept of trans
influence and covalency in the compound. Up to now, there has
never, to the best of our knowledge, been a combined experimen-
tal–theoretical study on the concept of trans influence in Ln com-
plexes with phosphoryl containing ligands.
of Ln–ligand
p bonding [12]. Although covalent bonding and trans
influence are well known in d-block metal complexes, Krogh-
Jespersen et al. have recently performed a combined theoretical–
experimental investigation to explain the concept of covalent
bonding and structural trans influence in Ln calcogenolate com-
plexes [13]. Based on the results of energy decomposition analysis
⇑
Corresponding author. Tel.: +98 21 82883443; fax: +98 21 8006544.
E-mail address: gholi_kh@modares.ac.ir (K. Gholivand).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.02.003

K. Gholivand, H.R. Mahzouni / Inorganica Chimica Acta 386 (2012) 8–12
9
Cl
Cl
LaCl3.7H2O
La
P
O
O
P
3
P
O
Acetonitrile
and
N
N
N
Cl
O
P
O
O
methanol
(6:1)
O
N
L
LaL3Cl3
O
Fig. 1. Schematic representation of the structure of ligand L and complex LaL3Cl3.
U(Ci) are the equivalent thermal parameters of the carbon atoms
to which corresponding H atoms are bonded. There is a void in
the asymmetric unit cell of LaL3Cl3 which is occupied by signifi-
cantly disordered water molecule. The best description of the
experimental data was achieved by disordering the water oxygen
atom over five positions (0.25:0.20:0.20:0.20:0.15). This disor-
dered oxygen was refined in isotropic approximation. The ORTEP
view of LaL3Cl3 is shown in Fig. 2 and crystallographic data are
summarized in Table 1.
2. Experimental
2.1. Materials and general methods
All chemicals were purchased commercially and used without
further purification. 1H, 13C and 31P NMR spectra were recorded
on a Bruker Avance DRX 500 spectrometer. 1H and 13C chemical
shifts were determined relative to an internal TMS standard, and
31P chemical shifts relative to 85% H3PO4 as external standard.
Infrared (IR) spectra were recorded on a Shimadzu model IR-60
spectrometer using KBr pellets. Elemental analysis was performed
on a Heraeus CHN-O-RAPID analyzer. Melting points were ob-
tained with an Electrothermal instrument.
2.5. Computational details
Both the structures L and LaL3Cl3 were fully optimized in vac-
uum at the B3PW91 level without any symmetry constraint using
the GAUSSIAN 98 package [23]. The quasi-relativistic effective core
potential (RECP) of Dolg et al. [24] has been used to represent
the atomic core of La and the valence electrons were taken into ac-
count by the affiliated (7s6p5d)/[5s4p3d] basis set, adding an extra
set of f polarization function with exponent 0.591 [25]. Non-metal
atoms H, C, N, O, P and Cl were described by 6-311G⁄ standard ba-
sis sets in optimizations. Natural population analysis (NPA) [26]
has been performed at B3PW91/ECP/6-311+G⁄⁄ level to investigate
the charge distributions in the optimized geometries. At the same
level, the electron densities at the bcp of La–O and La–Cl bond
paths have been analyzed based on the Bader’s atoms in molecules
(AIM) theory [27,28]. Moreover, the La–L and La–Cl binding ener-
gies have been calculated at B3PW91/ECP/6-311+G⁄⁄ and corrected
for basis set superposition error (BSSE) based on the counterpoise
(CP) correction method of Boys and Bernardi [29].
2.2. Synthesis of diphenyl morpholinyl phosphinamide (L)
A solution of 2 mmol chlorodiphenylphosphine oxide in 20 ml
THF was added dropwise to a solution of 4 mmol morpholine in
20 ml THF at 0 °C. After 5 h stirring, the solvent was evaporated
in vacuum. The resulting dense oil was washed with n-hexane to
obtain a white powder. Yield: 67%, m.p. 96 °C. Anal. Calc. for
C
16H18NO2P (287.27): C 66.89, H 6.31, N 4.87. Found: C 66.97, H
6.43, N 4.69%. Selected IR data (KBr, cmÀ1): 1190 s (
P@O), 958 s
PAN). NMR (CD3CN, 298 K): 31P{1H} d = 27.25 (s) ppm. 1H
m
(m
d = 2.96 (m, 4H, CH2), 3.64 (m, 4H, CH2), 7.50 (m, 6H, Ar–H), 7.86
3
(m, 4H, Ar–H) ppm. 13C{1H} d = 45.76 (s), 67.66 (d, JPC = 6.8 Hz),
128.99 (s), 129.70 (d), 132.90 (d), 133.16 (d) ppm.
2.3. Synthesis of complex LaL3Cl3
A solution of 3 mmol ligand L in 30 ml acetonitrile was added to
5 ml hot methanolic solution of 1 mmol LaCl3Á7H2O. The mixture
was stirred for 12 h. Then, the solution was left yielding crystals
suitable for X-ray diffraction. Yield: 65%, m.p. 185 °C. Anal. Calc.
for C48H56Cl3LaN3O7P3 (1125.13): C 51.24, H 5.01, N 3.73. Found:
C 51.43, H 4.93, N 3.66%. Selected IR data (KBr, cmÀ1): 1130 s
3. Results and discussion
3.1. Inverse trans influence in the crystal structure
As shown in Fig. 2, two ligands L1 and L2 are coordinated to the
La3+ cation in trans position with respect to each other in distorted
mer-octahedral complex LaL3Cl3, while the third ligand (L3) is trans
to the chloride (Cl2) counterion. Selected structural parameters are
summarized in Table 2. The La–O–P angle is 174.16(14)° for coor-
dinated ligand L3. The linearity of the La–O–P unit decreases for
two coordinated ligands L1 and L2, probably due to the steric
crowding and ligand–ligand repulsions, while their coordination
angle are 153.24(13)° and 152.37(13)°, respectively.
(mP@O), 964 s (m
PAN). NMR (CD3CN, 298 K): 31P{1H} d = 29.60 (s)
ppm. 1H d = 2.94 (m, 12H, CH2), 3.64 (m, 12H, CH2), 7.43 (m, 12H,
Ar–H), 7.55 (m, 6H, Ar–H), 7.98 (m, 12H, Ar–H) ppm. 13C{1H}
3
d = 45.99 (s), 67.51 (d, JPC = 6.9 Hz), 129.84 (s), 129.94 (s), 133.49
(s), 133.60 (d) ppm.
2.4. Crystal structure determination
Based on the concept of trans influence, metal–ligand bonds
trans to anions are expected to be longer than the same bonds that
are trans to neutral donor ligands. As well, anions trans to anions
are further from the metal atom than anions trans to neutral
donors. For instance, Krogh-Jespersen et al. have shown that the
structural trans influence makes the Yb–S bond trans to neutral
O-donor THF shorter than the same bond trans to C6F5SÀ anion in
X-ray data were collected on a Bruker SMART APEX2 CCD area
detector with graphite monochromated MoK
a
radiation
(k = 0.71073 Å) and refined by full-matrix least-squares methods
against F2 with SHELXL-97 [22]. All the hydrogen atoms in compound
LaL3Cl3 were refined in isotropic approximation within riding
model with the Uiso(H) parameters equal to 1.2 Ueq(Ci) where

10
K. Gholivand, H.R. Mahzouni / Inorganica Chimica Acta 386 (2012) 8–12
Fig. 2. Thermal ellipsoid of LaL3Cl3 at 50% probability. The hydrogen atoms have been omitted for clarity.
Table 1
Crystallographic data for compound LaL3Cl3.
Table 2
Selected geometrical parameters for solid state and gas phase structures of complex
Compound
LaL3Cl3
LaL3Cl3, and calculated binding energies at B3PW91/ECP/6-311+G⁄⁄
.
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C48H56Cl3LaN3O7P3
1125.13
triclinic
Bonds
d (Å)
Bond order
X-ray DFT
D )
Ea (kcal molÀ1
X-ray
DFT
ꢀ
P1
La–O1; trans to O2 2.407(2)
La–O2; trans to O1 2.402(2)
La–O3; trans to Cl2 2.390(2)
2.439 0.0620 0.0642 À28.1
2.444 0.0621 0.0634 À28.5
2.477 0.0591 0.0544 À22.8
11.0775(6)
14.2002(8)
17.6205(9)
95.9360(10)
95.4040(10)
105.5220(11)
2634.8(2)
2, 1.418
1.103
b (Å)
c (Å)
La–Cl1; trans to Cl3 2.7642(8) 2.797 0.1937 0.2040 À108.3
La–Cl2; trans to O3 2.7661(9) 2.755 0.2012 0.2005 À110.3
La–Cl3; trans to Cl1 2.7632(9) 2.843 0.2008 0.1768 À108.8
a
(°)
b (°)
P1–O1
P2–O2
P3–O3
P = O (free ligand)
Angles (°)
1.504(2)
1.501(2)
1.497(2)
–
1.522 1.0178 1.0152
1.522 1.0139 1.0144
1.516 1.0160 1.0335
–
–
–
–
c
(°)
V (Å3)
Z, Dcalc (Mg mÀ3
)
1.495
–
Absorption coefficient (mmÀ1
F(000)
)
X-ray/DFT
1148
Crystal size (mm)
h range (°)
Limiting indices (h, k, l)
0.17 Â 0.16 Â 0.14
La–O1–P1
La–O2–P2
La–O3–P3
153.24(13)/154.02
152.37(13)/155.32
174.16(14)/177.44
2.34–23.13
À15 6 h 6 15
À19 6 k 6 19
À24 6 l 6 24
31629/13917 [Rint = 0.0666]
13917/0/597
0.979
R1 = 0.0466, wR2 = 0.0747
R1 = 0.0778, wR2 = 0.0842
0.750/À0.853
a
For the optimized structure.
Reflections collected/unique
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices
R indices (all data)
Largest difference in peak/hole
lengths is often greater than that of a trans influence [30]. However,
the difference in either the La–O or La–Cl distances indicates a
directional dependence of the bonding in La complex studied here.
3.2. Normal trans influence in the gas phase structure
complex mer-(C6F5S)3Yb(THF)3 [13]. Here, the La–Cl2 bond length
(trans to neutral ligand L3) is expected to be shorter than the La–
Cl1 and La–Cl3 distances. Conversely, the La–Cl distances vary in
a small range, while the La–Cl2 bond length (2.7661(9) Å) is slightly
longer than the La–Cl1 (2.7642(8) Å) and La–Cl3 (2.7632(9) Å) dis-
tances. It seems that the tendency of Cl2 anion to participate in
weak intermolecular CH. . .Cl2 (ꢀ2.79–2.84 Å) interactions may be
a reason for lengthening of the La–Cl2 distance. Such intermolecu-
lar interactions together with the lengthening of the La–Cl2 dis-
tance may cause the La–L3 distance (trans to Cl2) to be shorter
than the La–L1 and La–L2 distances. Accordingly, the La–O3 bond
length (2.390(2) Å) is shorter than the La–O1 (2.407(2) Å) and
La–O2 (2.402(2) Å) distances. The weak intermolecular interactions
may be responsible for the inverse trans influence of Cl2 in the solid
state structure of mer-La(L)3Cl3. Similar findings has been previ-
ously reported for transition metal chemistry, where it has been
noted that the effect of intermolecular interactions on the bond
The geometries of complex LaL3Cl3 and free ligand (L) were
fully optimized in the gas phase in order to investigate the elec-
tronic and structural aspects of the compounds in the absence of
packing effects and intermolecular interactions created by neigh-
boring molecules. The optimized structural parameters are pre-
sented in Table 2.
Unlike the crystal structure of compound LaL3Cl3, its gas phase
structure shows a normal trans influence, while the optimized the
La–O3 (trans to Cl2) distance (2.477 Å) is considerably longer than
La–O1 (2.438 Å) and La–O2 (2.444 Å) distances. As a result, the
binding energy decreases considerably from La–O1 and La–O2
bonds (28.1 and 28.5 kcal molÀ1) to La–O3 (22.8 kcal molÀ1). Com-
paring the calculated wiberg bond indices, it is clear that the La–O3
bond order (0.0544) is relatively smaller than the values calculated
for the La–O1 (0.0642) and La–O2 (0.0634) bonds. The small values
for wiberg bond indices indicate a predominant ionic character of

K. Gholivand, H.R. Mahzouni / Inorganica Chimica Acta 386 (2012) 8–12
11
the La–O interactions, while the calculated wiberg bond indices in
the range 0.1768–0.2040 (Table 2) are indicative of a partial cova-
lent character of the La–Cl interactions. The intramolecular interac-
tions are able to displace the Cl3 atom from the expected
octahedral position, while the O3–La–Cl3 angle (79.8°) deviates
significantly from the right angle. This may have an effect on the
strength of the La–Cl3 bond with a small wiberg bond order
(0.1768).
nated ligands L1 and L2 become longer than the same bond in coor-
dinated ligand L3. This is in good agreement with the trend in the
calculated P@O wiberg bond orders.
The La–Cl binding energies (108.3–110.3 kcal molÀ1) are ꢀ4
times larger than the La–O binding energies (22.8–28.5 kcal molÀ1),
indicating that the chloride counterions interact more strongly with
the metal cation in comparison with the coordinated ligands L.
Comparing the La–Cl interaction energies, it is found that the La–
Cl2 bond is slightly stronger than the La–Cl1 and La–Cl3 bonds,
since the La–Cl2 bond is trans to neutral ligand L3.
Based on the AIM theory, the electron density (q(r)), its
Laplacian ( 2q(r)) and electronic energy density (H(r)) at a bond
critical point (bcp) give us information about the strength and
Considering the natural electron configurations 5s25p66s15d2
and 5s25p66s05d0 (for La and La3+, respectively) the natural elec-
tron configuration 5s1.995p5.986s0.164f0.015d0.23 for La3+ can be rep-
resentative of the LMCT in complex LaL3Cl3, in which the metal 6s
and 5d orbitals are partially occupied. This is confirmed by the loss
of electron density from either the ligands L or chloride counteri-
ons. Table 3 shows that chloride counterions are more involved
in LMCT as compared with coordinated ligands L1–L3, while the
negative charge of chloride counterions decreases to the range
from À0.870 to À0.878 upon coordination. This supports the calcu-
lated natural charge of +2.605 for La. The small decrease in the cat-
ionic charge from +3 to the range of (+2.3)–(+2.8) has also been
attributed to the LMCT in some lanthanide complexes previously
studied [32–34], confirming the q(La) value as a result of partial
charge transfer in complex LaL3Cl3. It is well-known that the
charge transfer plays a significant role for relatively weak ligands
to interact with lanthanide cations [35]. Moreover, the partial occu-
pancy of 5d [13] and 6s [10,36] orbitals can be considered as the
source of partial covalency in LaL3Cl3.
characteristic of the bond. The large
q(r) value and values of
2q(r) < 0 and H(r) < 0 refer to the covalent bond, while small
q
(r) values and values of 2q(r) > 0 and H(r) > 0 are calculated
for closed-shell and ionic interactions [27,28]. The electronic en-
ergy density (H(r)) at a bcp, instead of 2q(r), is a more appropriate
index to characterize better a weak interaction. The electronic en-
ergy density is obtained from the equation H(r) = G(r) + V(r), where
G(r) and V(r) are the local kinetic and potential energy densities,
respectively. The energy density term G(r) is calculated based on
equation (1/4) 2q(r) = 2G(r) + V(r) [27].
The
bond paths have been calculated and summarized in Table 3. The
(r) value at the bcp of La–O3 bond is slightly smaller than that ob-
q
(r), 2q(r) and H(r) values at the bcp of La–O and La–Cl
q
tained for La–O1 and La–O2 bonds. The positive values of the term
H(r) in the range 0.0027–0.0033 e2a0À4 indicate that all of the La–O
bonds are essentially ionic in character. In the case of chloride coun-
terions, the small negative values (À0.0009 to À0.0029 e2a0À4) for
the term H(r) at the bcp of bond paths are indicative of a partial
covalent character of the La–Cl bonds, agreeing with the calculated
wiberg bond orders. It is noteworthy that the largest
q(r) value
(0.0455 ea0À3) is related to the bcp of La–Cl2 bond (trans to
La–O3 bond). This bond has the most negative value of La–Cl inter-
action energy (À110.3 kcal molÀ1). Moreover, the H(r) value at the
bcp of La–Cl2 bond is more negative than that obtained for La–Cl1
and La–Cl3 bonds, indicating a more covalent character of the for-
mer bond compared with the others. The concept of covalency in
the Ln–halogen bonding has been previously explained [31].
It is well known that the complexation of a ligand is followed by
some electronic and geometrical perturbations in the structure of
coordinated ligand. Polarization of the P@O bond in the electro-
static field of cation leads to increase in the negative charge local-
ized on the phosphoryl oxygen atom [6]. Moreover, the P@O bond
becomes longer upon O-coordination as a function of the metal–li-
gand distance [8]. Here, the natural electron configurations
3s0.863p1.94 (for P atoms) and 3s1.793p5.42 (for phosphoryl oxygen
atoms) confirm the polarization of P@O bonds in complex LaL3Cl3.
The calculated NBO charges are À1.224 (O1), À1.225 (O2) and
À1.215 (O3) for coordinated oxygen atoms, confirming that the li-
gands L1 and L2 are more polarized than L3 (trans to Cl2) in the
electrostatic field of La3+. Accordingly, the P@O bond in coordi-
4. Conclusions
In the present study, the structural and electronic aspects of
octahedral complex mer-[(C6H5)2(NC4H8O)P@O]3LaCl3 have been
investigated to explain the concept of trans influence and covalent
bonding. Solid state structure of the compound shows an inverse
trans influence of chloride anion, probably due to the intermolec-
ular interaction effects. On the contrary, the gas phase structure
of the compound shows a normal trans influence of chloride anion,
while Cl2 atom (trans to neutral phosphoryl donor L3) is closer to
the La atom in comparison with Cl1 and Cl3 atoms. The ligand L3
(trans to Cl2) has been less polarized by the electrostatic field of
cation. The results of NBO analysis revealed a significant LMCT,
leading to the electron loss from the valence orbitals of chlorine
atoms. Moreover, the presence of electronic population in La 5d
and 6s orbitals associated with the directional dependence of
La–Cl distances may indicate a partial covalent character of the
bonding in complex LaL3Cl3. All of these structural and electronic
perturbations have been confirmed by the values of La–ligand dis-
tances, binding energies, wiberg bond indices, electron densities
and electronic energy densities.
Acknowledgment
Table 3
NBO charge for coordinating atoms, and electron density (q
(r) in ea0À3), Laplacian of
electron density ( 2q(r) in ea0À5), kinetic energy density (G(r) in e2a0À4) and total
electronic energy density (H(r) in e2a0À4) calculated at B3PW91/ECP/6-311+G⁄⁄ for
La–ligand bond paths in the optimized structure of LaL3Cl3.
Financial supports of this work by Tarbiat Modares University
Research Council are gratefully acknowledged.
La–X bond
NBO charge (q)a
q(r)
2q(r)
G(r)
H(r)
Appendix A. Supplementary material
X@O1
X@O2
X@O3
X@Cl1
X@Cl2
X@Cl3
À1.224
À1.222
À1.216
À0.875
À0.870
À0.878
0.0483
0.0476
0.0439
0.0410
0.0455
0.0376
0.2192
0.2165
0.2024
0.1239
0.1347
0.1134
0.0521
0.0513
0.0473
0.0323
0.0366
0.0292
+0.0027
+0.0028
+0.0033
À0.0013
À0.0029
À0.0009
Supplementary material CCDC 817640 contains the supplemen-
tary crystallographic data for the compound C48H56Cl3LaN3O7P3.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
a
q(La) = +2.605 for complex LaL3Cl3 and q(OP) = À1.068 for free ligand L.

12
K. Gholivand, H.R. Mahzouni / Inorganica Chimica Acta 386 (2012) 8–12
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