Nitrogenbased analogues of the uranyl ion

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Nitrogen-based analogues of the uranyl ion
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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.
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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
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Possible analogues - 2

Stable Analogues of the Uranyl Ion Containing
the -NUN- Group D.R. Brown and R.G. Denning,
Inorg. Chem. 35 (1996) 6158
i.e.
How good is this analogy?
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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
U6 UO22 O24-
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In 1999.what was generally not agreed upon The
ordering of these four molecular orbitals
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? two questions Is this the correct orbital
ordering? If so, why is the su orbital so much
less stable than the others?
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Possible reasons for the destabilisation of the
su MO of UO22

• 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.

• The pushing from below mechanism. The su
  orbital is destabilised by a filled-filled
  interaction with the U 6p semi-core orbitals.

C.K. Jørgensen and R. Reisfeld, Struct. Bonding
50 (1982) 121
Figure 5.10 from The f elements by N.
Kaltsoyannis and P. Scott
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Molecular orbital energy level diagrams for UO22

N. Kaltsoyannis, Inorg. Chem. 39 (2000) 6009.
R.G. Denning, JPCA 111 (2007) 4125.
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Dont believe everything you read in textbooks.
Overlap between the uranium valence orbitals and
the oxygen p levels decreases in the order fs gt
fp gt dp gt ds
?
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A better qualitative molecular orbital energy
level diagram for UO22

Always include the actinide 6p orbitals in your
calculations!
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Molecular orbital energy level diagrams for UN2,
UON and UO22

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Molecular orbital energy level diagrams for
OUNPH33 and U(NPH3)24

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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
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Molecular orbital energy level diagrams for
OUNPH33 and U(NPH3)24 with U 6p in core and
valence

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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 uraniumelement p
  bonding MOs are in all cases more stable than the
  uraniumelement s bonding levels, and that UN
  bonding is significantly more covalent than UO.
• The UN bonds in OUNPH3 and U(NPH3)24 are
  longer and less covalent than their equivalents
  in UON and UN2, and the UN s bonding MOs are
  more stable than the UN p levels. This reversal
  of the UN s/p MO ordering with respect to UON
  and UN2 is due to a combination of two factors
• (a) stabilising NP(H) s contribution to the UN
  s MO(s)
• (b) increased UN distance which destabilises the
  UN 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.

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So how good is this analogy?
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
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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
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Comparison of UO22, UN2 and U(NMe)2I2(THF)2

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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 fragmentalthough 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