Steric effects in complexes of diphenyl(2-pyridyl)phosphine oxide with the uranyl ion. Synthetic, structural and theoretical studies

Article abstract of DOI:10.1016/j.poly.2014.12.047

Uranyl complexes containing diphenyl(2-pyridyl)phosphine oxide, with the formulae [UO₂(NO₃)₂({C₆H₅}₂POC₅H₄N)₂] (1) and [UO₂(DBM)₂({C

Full text of DOI:10.1016/j.poly.2014.12.047

Polyhedron 89 (2015) 116–121
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Steric effects in complexes of diphenyl(2-pyridyl)phosphine oxide with
the uranyl ion. Synthetic, structural and theoretical studies
b
b,
c
Bal Govind Vats a, S. Kannan a, , K. Parvathi , D.K. Maity , M.G.B. Drew
⇑
⇑
a Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
b Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400085, India
c Department of Chemistry, University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Uranyl complexes containing diphenyl(2-pyridyl)phosphine oxide, with the formulae [UO2(NO3)2({C6H5}2
POC5H4N)2] (1) and [UO2(DBM)2({C6H5}2POC5H4N)] (2) (where DBM = C6H5COCHCOC6H5), were prepared
and characterized by IR, NMR spectroscopic and elemental analysis methods. The structures of the
compounds were determined by X-ray diffraction methods and revealed a monodentate mode of bonding
for the ligand through the phosphine oxide oxygen atom to the uranyl group. The pyridyl nitrogen atom of
the ligand is uncoordinated. The uranyl group is surrounded by eight oxygen atoms in a hexagonal
bi-pyramidal geometry in 1 and seven oxygen atoms in a pentagonal bi-pyramidal geometry in 2.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 26 September 2014
Accepted 30 December 2014
Available online 17 January 2015
Keywords:
Diphenyl(2-pyridyl)phosphine oxide
Uranyl nitrate
Diketonate
Structures
Theoretical studies
1. Introduction
donor atoms with uranyl and lanthanide ions [9]. Our recent stud-
ies support [9c] the earlier observations that ligands forming five
In recent years the structural and coordination chemistry of
uranium has grown in interest due to the availability of new syn-
thetic strategies [1] and also interesting properties such as, selec-
tive ion-exchange, mixed valencies, ionic conductivity, enhanced
fluorescence, magnetic ordering and non-linear optical properties
exhibited by its complexes [2]. A basic understanding of the com-
plex chemistry of the uranyl ion is very important in facilitating
the design of new ligands for the selective separation of this ion
from irradiated nuclear fuel, seawater, nuclear plant effluents, bio-
logical and environmental samples [3–5]. Many multifunctional
phosphine oxide based ligands have been examined for their abil-
ity to separate actinides and lanthanides from high level nuclear
waste solutions [6] and their structures with actinide and lantha-
nide ions have also been reported [7]. The chemistry of mono
and bi functional phosphine oxides with the uranyl ion is well
documented. In all cases the phosphine oxide bonds through the
oxygen atom to the uranyl ion [8]. The chemistry of bi-functional
phosphine oxides with the uranyl ion shows different modes of
bonding for the ligand depending upon the stoichiometry and nat-
ure of other groups present in the metal coordination sphere [8c].
We are interested in the coordination and structural chemistry of
bi-functional ligands containing soft nitrogen and hard oxygen
membered metallocyclic rings with the metal ion are more subject
to steric control than corresponding ligands forming six membered
rings [10]. Diphenyl(2-pyridyl)phosphine oxide is a bi-functional
ligand having soft pyridine nitrogen and hard phosphine oxide
groups. The chemistry of this ligand with transition metal ions is
well reported [11] and it is found to act as either a monodentate
ligand, bonding via the phosphine oxide oxygen atom, or as a
bidentate chelating ligand, bonding via both the phosphine oxide
oxygen and pyridyl nitrogen atoms. However, no report on the
chemistry of this ligand with any of the 4f or 5f elements is
available in the literature to our knowledge. In continuation of
our interest in the chemistry of phosphine oxide with the uranyl
ion [8c,d,12a,b], we report herein the complex chemistry of
diphenyl(2-pyridyl)phosphine oxide with uranyl nitrate and
uranyl bis(b-dibenzoylmethanate) and have explained the stability
of the complex formed by using theoretical studies.
2. Materials and methods
2.1. General considerations
All reagents and solvents were of analytical grade and used as
received. IR spectra were recorded as Nujol mulls using a JASCO-
610 FITR spectrometer. 1H and 31P NMR spectra were recorded
using a Bruker AMX-300 spectrometer. The chemical shifts (d)
⇑
Corresponding authors.
E-mail addresses: skannan@barc.gov.in (S. Kannan), dkmaity@barc.gov.in
(D.K. Maity).
http://dx.doi.org/10.1016/j.poly.2014.12.047
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

B.G. Vats et al. / Polyhedron 89 (2015) 116–121
117
are reported in ppm. Electrospray ionization mass spectrometric
2.6. Crystal structure determinations
detection of positive ions in CH2Cl2 or CH3COCH3 was recorded
using a MicrOTOF Q-II instrument. The samples were introduced
into the source with a syringe pump. Nitrogen was employed as
both the drying and spraying gas with a source temperature of
180 °C. The cone voltage was set to 45 V, the voltage applied on
the capillary was 1162 kV and the sample solution flow rate was
Crystal data for 1 and 2 were measured on an Oxford Diffraction
X-Calibur CCD system at 150(2) K with Mo radiation
(k = 0.71073 Å). The crystals were positioned 50 mm from the
CCD. 321 Frames were measured with a counting time of 10 s. Data
analyses were carried out with the CRYSALIS program [14a]. The
K
a
5
l
L minꢀ1. The spectra were recorded for m/z values of 100–1000.
structures were solved using direct methods with the SHELXS97 pro-
gram [14b]. All non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms bonded to carbon atoms
were included in geometric positions and given thermal parame-
ters equivalent to 1.2 times those of the atoms to which they are
attached. Empirical absorption corrections were carried out using
the ABSPACK program [14c]. The structures were refined to conver-
gence on F2 using SHELXL97 [14b]. In the structure of 1, the nitrogen
atom of the pyridyl ring was disordered over two positions with a
refined ratio of 62:38. In the structure of 2, four of the aromatic
rings showed severe disorder and in each case two orientations
were included in the refinement with occupation factors x and
1 ꢀ x, with x refining to values close to 0.5. Selected crystallo-
graphic data for 1 and 2 are summarized in Table 1.
2.2. Synthesis of L
To a solution of diphenyl, 2-pyridyl phosphine (5 g, 19 mmol) in
benzene (50 mL), 30% H2O2 (2 mL) was added and stirred for 5 h.
This solution was dried over anhydrous sodium sulfate and filtered.
The filtrate on evaporation yielded a light yellow colored powder,
which was filtered, washed with hexane and dried (Yield. 85 %).
31P{1H} NMR (25 °C, CDCl3) d (ppm): 21.280. IR (cmꢀ1
) m: 1191
(P@O). Anal. Calc. for C17H14NPO: C, 73.1; H, 5.0; N, 5.0. Found:
C, 72.8; H, 4.8; N, 4.8%.
2.3. Synthesis of 1
To a solution of L (112 mg, 400 mmol) in methanol (20 mL),
solid [UO2(NO3)2ꢁ6H2O] (100 mg, 200 mmol) was added and stirred
for few minutes until all the solid dissolved to give a clear solution.
This solution was filtered and allowed to evaporate slowly at room
temperature. This process yielded a yellow crystalline solid, which
was filtered, washed with hexane and dried (yield 84 %). 31P{1H}
3. Results and discussion
3.1. Synthesis of the diphenyl(2-pyridyl)phosphine oxide ligand (L)
This ligand was prepared by the oxidation of the corresponding
phosphine with H2O2 in benzene. The IR spectrum shows the
presence of a P@O group (1191 cmꢀ1) in the synthesized ligand.
NMR (25 °C, CDCl3) d (ppm): 36.726. IR (cmꢀ1
) m: 1161 (P@O). Anal.
Calc. for C34H28N4P2O10U: C, 42.9; H, 2.9; N, 5.9. Found: C, 42.6; H,
2.8; N, 5.6%.
The 31P NMR spectrum shows
a
single resonance at
d = 21.28 ppm, which is ca. 25.18 ppm downfield compared to that
of the starting phosphine (d = ꢀ3.90 ppm). The CHN analyses
support the expected stoichiometry for the newly prepared ligand.
2.4. Synthesis of 2
To a solution of L (57 mg, 200 mmol) in methylene chloride
(20 mL), solid [UO2(DBM)2ꢁH2O] (150 mg, 200 mmol) was added
and stirred for few minutes until all the solid dissolved to give a
clear solution. This solution was filtered and layered with iso-
octane. The solution on slow evaporation yielded an orange colored
crystalline solid, which was filtered, washed with hexane and dried
3.2. Synthesis of the diphenyl(2-pyridyl)phosphine oxide, uranyl
nitrate complex (1)
The reaction of [UO2(NO3)2ꢁ6H2O] with the ligand {C6H5}2POC5
H4N in methanol yielded compound 1 (Scheme 1). The CHN analy-
ses revealed that the ratio of uranyl nitrate to ligand is 1:2. The IR
spectrum of 1 shows that the water molecules from the starting
compound [UO2(NO3)2ꢁ6H2O] are completely replaced by the ligand
and that the ligand is bonded through the phosphoryl oxygen atom
to the uranyl group. The observed frequency difference for the PO
(yield 90 %). 31P{1H} NMR (25 °C, CDCl3) d (ppm): 31.796. IR (cmꢀ1
: 1171 (P@O). Anal. Calc. for C47H36NPO7U: C, 56.7; H, 3.6; N, 1.4.
)
m
Found: C, 56.6; H, 3.7; N, 1.2%.
2.5. Theoretical methods
group (
D
m
PO = 30 cmꢀ1, where
D
mPO
=
mPO (free ligand)
ꢀ
mPO(coordinated))
Geometry optimization for the diphenyl(2pyridyl)phosphine
oxide ligand and complex 1 has been carried out by applying a
popular non-local correlated hybrid density functional, namely,
B3LYP. Gaussian type atomic basis functions, 6-31+G(d), were
adopted for the H, C, N and O atoms and for U atom very recently
suggested basis sets, SARC-ZORA [13a], were used for all the calcu-
lations. SARC-ZORA basis sets are segmented all-electron scalar
relativistic basis sets in which the coefficients of contracted GTOs
have been optimized for use with the ZORA scalar relativistic
Hamiltonian. These particular basis sets for U were obtained from
the Extensible Computational Chemistry Environment Basis Set
Database, Pacific Northwest National Laboratory [13b]. The
quasi-Newton–Raphson based algorithm has been applied to carry
out geometry optimization to locate the minimum energy struc-
ture in each case. Hessian calculations have also been carried out
to check the nature of the equilibrium geometry. A macroscopic
solvation effect of the solvent dichloromethane has been incorpo-
rated in the energy calculation through the polarizable continuum
model (PCM). All these calculations have been carried out with the
GAMESS suite of ab initio programs [13c].
Table 1
Crystal data refinement of compounds 1 and 2.
1
2
Empirical formula
C
34H28N4O10 P2U
C47H36NO7PU
monoclinic
P21/c
17.9927(6)
11.1401(3)
20.5688(6)
101.982(3)
4033.0(2)
4
1.640
4.118
8732/6817
6817/0/398
1.180
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
9.726(3)
11.074(3)
15.9365(14)
93.180(19)
1713.9(7)
2
1.846
4.893
4935/3357
3357/0/233
1.011
b (°)
V (cm3)
Z
qcalc (g cmꢀ3
)
l
(mmꢀ1
)
Reflections collected/unique
Data/restraints/parameters
Goodness of fit (GOF) on F2
Final R1 indices [I > 2
wR2 indices (all data)
r(I)]
0.0606
0.1080
0.0695
0.1300
w = 1/[
2, where P = (F2o + 2Fc2)/3.
r r
2(F2o) + (0.0062P)2 + 0.000P] for 1, w = 1/[ 2(F2o)+(0.0176P)2 + 24.1300P] for

118
B.G. Vats et al. / Polyhedron 89 (2015) 116–121
two diphenyl(2-pyridyl)phosphine oxide ligands form the equato-
rial hexagonal plane. The UO6 atoms in the equatorial plane show
an r.m.s deviation of 0.029 Å. The two uranyl oxygen atoms occupy
the axial positions.
This type of coordination is similar to that observed in
phosphine oxide, amide, pyrolidone compounds with uranyl
nitrate, such as [UO2(NO3)2(OPPh3)2] [8], [UO2(NO3)2(amide)2]
[15a], [UO2(NO3)2(N-cyclohexyl,2-pyrrolidone)2] [15b] and
[UO2(NO3)2(1,3-dimethyl,2-imidazolidone)2] [15c]. The average
U–O(phosphine oxide) distance (2.350(4) Å) in 1 is comparable in mag-
nitude with those of earlier reported uranyl nitrate-phosphine
compounds, such as [UO2(NO3)(TBPO)2] (TBPO = tributyl phos-
phine oxide) [8a], [UO2(NO3)2(DPEPA)2] (2.347(5), 2.379(4) Å)
(DPEPA = diphenyl-N,N-ethylphosphine amide) [8b], [UO2(NO3)2
(TPPA)2] (2.340(3) Å) (TPPA = tripiperidine phosphine oxide) [8b],
[UO2(NO3)2(DPPMO2)] (2.386(9) Å) [8c], [UO2(NO3)2(CMPO)]
(CMPO = carbamoylmethyl phosphine oxide) (2.38(2) Å) [7c],
[UO2(NO3)2(DPPFO2)] (2.352(2) Å) [8d], [UO2(DPPMO2)2(OPPh3)]
(2.343(2) Å) [12a] and [UO2(OPPh3)4]2+ (2.2907–2.3034(17) Å)
[12b–c]. The observed average U–O(NO3) (2.534(5) Å) [8,12] and
U–O(uranyl) (1.758(4) Å) [8,12,15,16] distances are normal. The
angles subtended at the metal atom show that the uranium atom
has a slightly distorted hexagonal bi-pyramidal geometry.
It is interesting to note here that the ligand forms a 2:1 complex
with uranyl nitrate and bonds through the phosphine oxide oxygen
atoms to the metal center, leaving the pyridyl nitrogen atom
un-coordinated. The chelating mode of bonding via the phosphine
oxide oxygen and pyridyl nitrogen atoms is found for this ligand
with transition metal ions, forming a five membered metallocyclic
ring. We have also reported recently that the pyrazolyl urea ligand
[9] forms a five member metallocyclic ring with the uranyl ion via
the pyrazolyl nitrogen and carbamoyl oxygen atoms. In order to
find out the reason for not forming the five membered metallocy-
clic ring or not acting as a chelating ligand with the uranyl ion,
theoretical studies were carried out. Theoretical studies (see
below) clearly reveal that steric effects play an important role in
deciding the mode of bonding for this ligand.
Scheme 1. Synthesis of the ligands and its uranyl complexes.
shows that the phosphoryl oxygen atom is bonded to the uranyl
ion directly [8b–e,12a–b]. This difference is comparable in magni-
tude with those observed in [UO (NO3)2(DPPMO2)] (DPPMO =
2
bis(diphenylphosphino)methane
dioxide)
[8c],
[UO2(NO3)2
(DPPFO2)] (DPPFO = 1,10-bis(diphenylphosphino)ferrocene dioxide)
[8d], [UO2Cl2(DPPFO2)] [8e], [UO2(DPPMO2)2 (OPPh3)](BF4)2 [12a]
and [UO2(OPPh3)4](OTf)2 [12b]. The 31P NMR spectrum of 1 shows
a single resonance at d 36.73 ppm, which is ca. 15.45 ppm down-
field compared to that of the free ligand. This observation provides
further evidence that the phosphoryl oxygen atom is coordinated to
the uranyl ion. The ESI-MS spectrum of 1 in acetonitrile shows the
presence of peaks at m/z of 581.5 [(UO2L3(H2O)3+2H)2+, 100%] and
302 [(UO2L(H2O)3+H)2+, 100%], indicating that the metal ligand
bond is retained in solution. It is apparent from the IR, NMR spectral
and CNH analysis results that the ligand acts as a monodentate
ligand and bonds through the phosphoryl oxygen atom to the
uranyl ion. The structure of 1 has been determined by single crystal
X-ray diffraction methods and confirms the spectral and analysis
results.
3.3. Structure of [UO2(NO3)2({C6H5}2POC5H4N)2] (1)
The structure of 1 is shown in Fig. 1, and selected bond
distances and angles are given in Table 1. The structure consists
of a centrosymmetric [UO2(NO3)2({C6H5}2POC5H4N)2] moiety, in
which the uranium atom is surrounded by eight oxygen atoms in
a hexagonal bi-pyramidal geometry. Four oxygen atoms of the
two bidentate nitrate groups, together with two oxygen atoms of
3.4. The diphenyl(2-pyridyl)phosphine oxide uranyl
bis(dibenzoylmethanate) complex (2)
The reaction of (C6H5)2POC5H4N with [UO2(C6H5COCHCOC6
H5)2ꢁ2H2O] yielded compound 2 (Scheme 1). The CHN analyses
Fig. 1. The centrosymmetric structure of 1. The nitrogen atom is disordered over two sites N46 and C26 with occupation factors of 0.62 and 0.38 respectively.

B.G. Vats et al. / Polyhedron 89 (2015) 116–121
119
Table 2
revealed that the ratio of ligand to uranyl bis(dibenzoylmethanate)
Important bond lengths (Å) and angles (°) for 1 and 2.
is 1:1. The IR spectrum shows that the water molecules from
the starting compound [UO2(C6H5COCHCOC6H5)2ꢁ2H2O] are
completely replaced by the ligand and furthermore the observed
1
U1–O1
U1–O2
P1–O1
2.350(5)
1.758(4)
1.521(4)
U1–O11
U1–O13
2.530(5)
2.538(5)
frequency difference for the phosphoryl group (
where PO(coordinated)) is consistent with the
Dm ,
PO = 20 cmꢀ1
D
mPO
=
mPO(free ligand) ꢀ
m
O2–U1–O1
O2–U1–O13
O1–U1–O13
P12–O1–U1
89.7(2)
90.3(2)
114.6 (2)
159.6(3)
supposition that the phosphoryl group is bonded to the uranyl
O2–U1–O11
O1–U1–O11
O11–U1–O13
89.8(2)
64.5(2)
50.1(2)
ion directly.
The 31P NMR spectrum of 2 shows a single resonance at
31.79 ppm, which is deshielded about ca 10.51 ppm with respect
to the free ligand, indicating that the metal ligand bond persists
in solution. The structure of 2 has been determined by single
crystal X-ray diffraction methods and confirms the spectral and
analysis results.
2
U1–O1
U1–O2
U1–O15
U1–O3
O1–U1–O2
O31–U1–O3
O11–U1–O15
P3–O3–U1
O3–U1–O11
1.770(5)
1.775(5)
2.381(6)
2.389(5)
178.7(2)
73.9(2)
69.6(2)
167.4(3)
142.9(2)
U1–O31
U1–O35
U1–O11
P3–O3
O31–U1–O35
O3–U1–O15
O11–U1–O35
O3–U1–O35
2.357(6)
2.352(6)
2.329(5)
1.487(5)
70.1(2)
74.3(2)
72.9(2)
143.9(2)
3.5. Structure of [UO2(C6H5COCHCOC6H5)2(C6H5)2POC5H5N] (2)
The structure of 2 is shown in Fig. 2 together with the number-
ing scheme and selected bond distances and angles are given in
Table 2. The structure shows that the uranyl ion is bonded to
two C6H5COCHCOC6H5 groups and one diphenyl(2-pyridyl)
phosphine oxide ligand to give a coordination number of seven.
The diphenyl(2-pyridyl)phosphine oxide ligand acts as a monoden-
tate ligand and is bonded through the phosphine oxygen atom
to the uranyl ion. Four oxygen atoms from two bidentate
C6H5COCHCOC6H5 groups and one oxygen atom from the phosphine
oxide ligand form the equatorial plane and together with two
oxygen atoms of the uranyl ion form a pentagonal bi-pyramidal
geometry around the uranium(VI) ion.
The five oxygen atoms in the equatorial plane show an r.m.s
deviation of 0.102 Å. Similar structures are also observed in phos-
phine oxide, sulfoxide, ketone, N-oxide and amide compounds
with uranyl bis(b-diketonates), viz: [UO2(DBM)2(OPPh3)] (DBM =
dibenzoylmethanide) [17a], [UO2(TTA)2TIBP] (TTA = theonoyltri-
fluoroacetate; TIBP = triisobutylphosphate) [17b], [UO2(DBM)2
(C6H5SOCH3)] [17c], [UO2(TTA)2(C5H5NO)][17d], [UO2(DBM)2(cam-
phor] [17e], [UO2(DBM)2(C4H9CON{C4H9)2)] [17f] and [UO2(DBM)2
(iC3H7CON{iC3H7)2] [15a]. The observed U–O phophine oxide bond
distance (2.389(5) Å) is comparable with those of the earlier
reported uranyl bis (b-diketonate)-phosphine oxide compounds
[17a,b,18]. All other bond distances and angles are normal [17,18].
3.6. Theoretical studies on complexes of diphenyl(2-pyridyl)phosphine
oxide with uranyl nitrate
To examine the feasibility of the binding of diphenyl(2-
pyridyl)phosphine oxide with the uranyl ion, either as a chelating
ligand binding through both the phosphine oxide and pyridyl
nitrogen atoms or as a monodentate ligand binding through phos-
phine oxide oxygen atom, quantum chemical calculations have
been carried out. Full geometry optimization of the possible
complexes has been performed in the gas phase as well as in
dichloromethane solution following
a macroscopic solvation
model. The minimum energy structures predicted in solution are
presented in Fig. 3(a and b).
In the chelated complex (3a), the calculated U–O(phosphoryl)
and U–N (pyridyl) bond distances are 2.52 and 3.06 Å, respectively.
The large U–N bond distance in 3a appears to be due to steric hin-
drance between the pyridine ring and the adjacent nitrate groups.
The calculated U–O (phosphoryl) bond length of 2.54 Å in the
monodentate structure (3b) is slightly longer as compared with
that obtained for the bidentate chelating structure (3a). Note that
these bond distance parameters are about 0.1 Å shorter as com-
pared to those calculated in the gas phase. To calculate the binding
energy for the chelating mode of bonding between the ligand and
UO2(NO3)2, the energy of the whole complex (3a) was calculated
by increasing the distance between the ligand and the UO2(NO3)2
moiety to generate a dissociation curve, and thus the binding
energy is calculated as 37.7 kcal/mol. In the case of the 2:1 mono-
dentate complex (3b), the dissociation curve is generated by calcu-
lating the energy by increasing the distance between the U and O
atoms of both the ligands. The total binding energy is thus calcu-
lated as 51.2 kcal/mol (for two ligands with the monodentate
mode of bonding) and the binding energy per ligand is calculated
as 25.6 kcal/mol. As expected, the binding energy calculation
clearly revealed that the chelating mode of bonding via the phos-
phoryl oxygen and pyridyl nitrogen atoms (37.7 kcal/mol) is more
stable than that of the monodentate mode of bonding via the phos-
phoryl oxygen atom (25.6 kcal/mol) by 12.1 kcal/mol. However,
between the 2:1 complex (3b) with two monodentate ligands
(51.2 kcal/mol) and the 1:1 complex (3a) with one chelating ligand
(37.7 kcal/mol), the former is more stable than that of later by
13.5 kcal/mol. This shows clearly that 2:1 complex with two
monodentate ligand is energetically more favorable than that of
Fig. 2. Structure of 2. Four of the aromatic rings are disordered over two
orientations. Only those with the higher occupancy are shown.

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B.G. Vats et al. / Polyhedron 89 (2015) 116–121
Fig. 3. (a) Optimized structures for [UO2(NO3)2L] and (b) [UO2(NO3)2ꢁ2L].
1:1 complex with one chelating ligand. This observation is in
agreement with our experimental results.
the basic nature of these orbitals is quite similar in both
complexes in that the phenyl orbitals of the ligand participate
p
The HOMOs and LUMOs of the [UO2(NO3)2L] and [UO2(NO3)2ꢁ
in the HOMO, whereas the LUMO is comprised mainly of the
U[5fz(x2ꢀy2)] orbital.
2L] complexes are shown in Fig. 4(a–d). It is observed that
Fig. 4. (a) HOMO of [UO2(NO3)2L], (b) LUMO of [UO2(NO3)2L], (c) HOMO of [UO2(NO3)2ꢁ2L] and (d) LUMO of [UO2(NO3)2ꢁ2L].

B.G. Vats et al. / Polyhedron 89 (2015) 116–121
121
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The structural studies on the complexes of diphenyl
(2-pyridyl)phosphine oxide with uranyl nitrate and uranyl
bis(dibenzoylmethanate) show a monodentate mode of bonding
for the ligand. Theoretical studies reveal that the steric effects play
an important role in deciding the mode of bonding for the ligand.
The 2:1 complex [UO2(NO3)2ꢁ2L], in which the ligand acts as a
monodentate ligand, is energetically more stable by 13.5 kcal/mol
than the 1:1 complex [UO2(NO3)2ꢁL], in which the ligand acts as a
bidentate chelating ligand.
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We wish to thank Dr. S. K. Aggarwal, Associate Director, Radio-
chemistry and Isotope Group and Head, Fuel Chemistry Division for
his support. We also wish to thank, Shri. Pranaw Kumar for the
EI-MS Spectra, Dr. V. K. Jain, Chemistry Division, BARC for NMR
spectra, and EPSRC (UK) and the University of Reading, United
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CCDC 1025071 and 1025072 contain the supplementary crys-
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charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
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