Nitrogen-based analogues
of the uranyl ion
Nik Kaltsoyannis
Department of Chemistry
University College London
Introduction
Uranyl UO22+ is the most common functional unit in the chemistry of U(VI).
Imido ligand NR2- is isoelectronic with the oxo O2- ligand, and the two groups can
often be interchanged in transition metal complexes.
The alkyl or aryl R group provides a variable unavailable in oxo chemistry 
synthesis of the isoelectronic imido analogues of uranyl is highly desirable.
However, synthesis of imido uranyl analogues has proved very difficult. In 1996
Denning speculated that the isolation of U(NR)2 is not possible because U(VI) is
too oxidising.
Possible analogues - 1
“Synthesis and structure of the first U(VI) organometallic complex”
D.S.J. Arney, C.J. Burns and D.C. Smith, JACS 114 (1992) 10068
Possible analogues - 2
“Stable Analogues of the Uranyl Ion Containing the -N=U=N- Group”
D.R. Brown and R.G. Denning, Inorg. Chem. 35 (1996) 6158
PR3
PR3
2Cl
N
Cl
Cl
U
Cl
N
PR3
O
Cl
Cl
O
2+
Cl
i.e.
U
Cl
4+
N
O
U
U
N
O
PR3
How good is this analogy?
On the valence electronic structure of UO22+
In 1999….what was generally agreed upon:
UO22+ has 12 valence electrons (coming from the oxygen 2p and uranium 5f, 6d and 7s orbitals)
These electrons are accommodated in four molecular orbitals, of pg, pu, sg and su symmetry
6d anti-bonding (sandp)
6d non-bonding ()
U(6d)
5f anti-bonding (sandp)
U(5f)
5f non-bonding (and)
O(2p) combinations
(6 in total)
su
6 bonding orbitals
(filled in UO22+)
U6+
sg
pg
pu
UO22+
O24-
In 1999….what was generally not agreed upon:
The ordering of these four molecular orbitals
Reference
Computational method
Molecular orbital ordering
Cornehl et al., Angew. Chem.
Int. Ed. Engl. 35 (1996) 891.
CCSD/quasi-relativistic
pseudopotentials
pg < sg < pu < su
K.G. Dyall, Mol. Phys. 96
(1999) 511.
DHF/all electron
pg < su < sg < pu
Zhang et al., J. Phys. Chem. A
103 (1999) 6880.
CISD/relativistic ECPs
pg < su < pu < sg
De Jong et al., J. Mol. Struct.
(THEOCHEM) 458 (1999) 41.
DHF+CCSD(T)/all electron
pg < pu ~ sg < su
Summary of experimental studies
(from R.G. Denning, Struct. Bonding 1992, 79, 215.)
su (HOMO)
sg
pu+pg
(order not certain)
 two questions:
Is this the correct orbital ordering?
If so, why is the su orbital so much
less stable than the others?
Possible reasons for the destabilisation of the su MO of UO22+
1
Overlap between the U fs and O 2p orbitals is
small because the position of the angular
nodes of the fs orbital gives rise to extensive
overlap cancellation in regions of different
phase.
z
O
y
2
The “pushing from below”
mechanism. The su orbital is
destabilised by a filled-filled
interaction with the U 6p
semi-core orbitals.
U
x
O
Figure 5.10 from The f elements by
N. Kaltsoyannis and P. Scott
C.K. Jørgensen and R. Reisfeld,
Struct. Bonding 50 (1982) 121
eV
Molecular orbital energy level diagrams for UO22+
-19
u
100% U 5f
-20
u
100% U 5f
-21
su (HOMO)
56% U 5f, 8% U 6p
34% O 2p, 2% O 2s
-22
49% U 5f
47% O 2p, 3% O 2s
-23
14% U 6d, 2% U 7s
76% O 2p, 8% O 2s
36% U 5f
63% O 2p
-24
-25
19% U 6d
80% O 2p
sg
13% U 6d, 2% U 7s
77% O 2p, 8% O 2s
pu
35% U 5f
64% O 2p
su (HOMO)
sg
pu
pg
pg
U 6p in core
20% U 6d
79% O 2p
U 6p in valence
N. Kaltsoyannis, Inorg. Chem. 39 (2000) 6009. R.G. Denning, JPCA 111 (2007) 4125.
Don’t believe everything you read in textbooks….
Overlap between the uranium valence orbitals and
the oxygen p levels decreases in the order
fs > fp > dp > ds
z
O

y
U
x
O
A better qualitative molecular orbital energy
level diagram for UO22+
Always include the actinide 6p
orbitals in your calculations!
eV
Molecular orbital energy level diagrams for UN2, UON+ and UO22+
+2
1u U f
1u U f
1 U f
+1
3su U-N s
0
3su U-O s
6s U-N s
3sg U-N s
3sg U-O s
43% U d/f
-1
-2
41% U f
58% N p
34% U d
66% N p
2pu U-N p
1pg U-O p
1pg U-N p
22% U d/f
75% O p
2p U-O p
1.62
-0.67
1.62
U
1.734
+1.34
Dh
20% U d
79% O p
5s U-O s
-3
N
35% U f
64% O p
2pu U-O p
3p U-N p 55% N p
N
N
1.659
-0.50
0.90
U
1.751
+2.12
Cv
O
-0.62
+
O
-0.44
0.94
U
1.716
+2.88
Dh
O
2+
Molecular orbital energy level diagrams for OUNPH33+ and U(NPH3)24+
eV
+2
5a2u+1a1u U f
1a2+8a1 U f
+1
0
(HOMO)
3eu U-N p/P-H s
4e U-N p/P-H s
2eg U-N p/P-H s
7a1 U-O s
#
O lone pair/p
between N p and
P-H s
4a2u U-N-P-H s
-1
2eu##
3e U-O p
-2
(HOMO)
2e#
1eg##
6a1 O-U-N-P-H s
U-N p/p
between U-Np
and P-H s
##
4a1g U-N-P s
-3
1.42
0.92
0.57
1.36
1.711Å 1.824Å 1.781Å
O
U
N
PH3
-0.44 +3.06 -0.66 -0.59
3+
0.59
1.823Å 1.822Å
PH3N
U
N
PH3
+3.08 -0.66 -0.63
4+
Why is s below p in OUNPH33+ and U(NPH3)24+?
4e U-N p
6a1 O-U-N-P-H s
Walsh diagram for elongation of the U-N bond in OUNPH33+
eV
Molecular orbital energy level diagrams for OUNPH33+
and U(NPH3)24+ with U 6p in core and valence
+2
+1
0
1a2+8a1 U f
-2
5a2u+1a1u U f
1a2+8a1 U f
*O lone pair/p
between N p and
P-H s
**U-N p/p
between U-Np
and P-H s
4e U-N p/P-H s
4e U-N p/P-H s
3eu U-N p/P-H s
3eu U-N p/P-H s
2eg U-N p/P-H s
2eg U-N p/P-H s
7a1 U-O s
-1
5a2u+1a1u U f
7a1 U-O s
4a2u U-N-P-H s
3e U-O p
3e U-O p
2e*
2e*
6a1 O-U-N-P-H s
6a1 O-U-N-P-H s
-3
U 6p in valence
U 6p in core
0.92
1.711
O
-0.44
4a2u U-N-P-H s
1.42
1.824
0.57
1.781
U
N
+3.06 -0.66
C3v
PH3
-0.59
3+
2eu**
2eu**
1eg**
1eg**
4a1g U-N-P s
4a1g U-N-P s
U 6p in valence
U 6p in core
PH3N
1.36
1.823
0.59
1.822
U
N
+3.08 -0.66
D3d
PH3
-0.63
4+
Summary
Density functional theory calculations on UO22+ confirm the valence MO ordering
proposed by Denning (on the basis of experimental data) and indicate that the su HOMO
is destabilised with respect to the other valence MOs on account of a filled-filled
interaction with the uranium 6p semi-core orbitals (the pushing from below mechanism).
Comparison of the isoelectronic series UO22+, UON+ and UN2 indicates that the uranium–
element p bonding MOs are in all cases more stable than the uranium–element s
bonding levels, and that U–N bonding is significantly more covalent than U–O.
The U–N bonds in OUNPH3+ and U(NPH3)24+ are longer and less covalent than their
equivalents in UON+ and UN2, and the U–N s bonding MOs are more stable than the U–
N p levels. This reversal of the U–N s/p MO ordering with respect to UON+ and UN2 is
due to a combination of two factors:
(a) stabilising N–P(–H) s contribution to the U–N s MO(s)
(b) increased U–N distance which destabilises the U–N p bonding levels.
As with UN2 (and UO22+) the ds MO of U(NPH3)24+ is much more stable than the fs. This
is partly due to the destabilising influence of the pushing from below mechanism on the fs
MO, which is found to operate in the iminato systems to a similar extent as in UO22+.
PR3
4+
2+
So how good is this analogy?
N
O
U
U
N
O
PR3
“It is certainly correct of Denning to describe uranium bis iminato
complexes as structural analogs of the uranyl ion, it is not clear that the
analogy can be fully extended to the electronic structure”
N. Kaltsoyannis, Inorg. Chem. 39 (2000) 6009
Never say never….
T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer,
E.R. Batista and P.J. Hay, Science 310 (2005) 1941
T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer,
E.R. Batista and P.J. Hay, JACS 128 (2006) 10549
eV
Comparison of UO22+, UN2 and U(NMe)2I2(THF)2
+2
+1
0
(HOMO)
-1
-2
41% U f
58% N p
34% U d
66% N p
3su U-N s
U-N p
3sg U-N s
32% U f
27% U f
39% U f, 23% U d
24% U d
2pu U-N p
3su U-O s
(HOMO)
3sg U-O s
35% U f
64% O p
2pu U-O p
U-N s 24% U f
1pg U-O p
20% U d
79% O p
1pg U-N p
-3
U-N s
10% U d
blue = Mulliken
red = NBO
N
1.734Å
U
N
+1.34
-0.67
MeN
U
1.844Å
+1.50
+1.27
NMe
O
U
1.716Å
+2.88
+2.84
O
-0.44
2+
“Overall, we can say that the U-N bonding orbitals in U(NMe)2I2(THF)2 are
of the same type as those of the UO22+ fragment…although the ordering is
different. This difference in order is an indication that the “pushing from
below” mechanism proposed for uranyl does not exert a strong influence in
the present case due perhaps to a smaller involvement of the uranium 6p
orbital in the binding MOs”
T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista
and P.J. Hay, JACS 128 (2006) 10549