Ligand Exchange Reactions involving Hypervalent Iodine Compounds
3c4e (hypervalent) bond
+ + I
Formation of hypervalent (HV) bonds ligand
L I L + Nu- L I Nu + L
L I Nu + Nu- Nu I Nu + L
(1) Musher, J. I. The Chemistry of Hypervalent Molecules. Angew. Chem. Int. Ed. 1969, 8, 54-68.
(2) Martin, J. C. "Frozen" Transition States: Pentavalent Carbon et al. Science 1983, 221, 509-514
(3) Sandin, R. B. Organic Compounds of Polyvalent Iodine. Chem. Rev. 1943, 32, 249-276.
(4) Banks, D. F. Organic Polyvalent Iodine Compounds. Chem. Rev. 1966, 66, 243-266.
(5) Stang, P. J.; Zhdankin, V. V. Organic Polyvalent Iodine Compounds. Chem. Rev. 1996, 96, 1123-
The first two references provide good introduction to hypervalency (i.e., the formation of more
chemical bonds by atoms of main group elements than expected, based on the number of unpaired
electrons). References 3-5 are specific for hypervalent iodine compounds, including structure and
As soon as iodine was discovered in the first quarter of 19th century, many of its
compounds were synthesized and studied. Although in many of its compounds (HI, KI, C2H5I)
iodine is monovalent, a number of compounds containing tri-, penta-, or even heptavalent iodine
are known, both inorganic (e.g., ICl3, I2O5, HIO3, and KIO4), and organic (e.g., C6H5ICl2). In fact,
numerous organic polyvalent iodine compounds have been synthesized, including
iodosylarenes (ArIO), (diacyloxyiodo)arenes (ArI(O2CR)2), and iodoxyarenes (ArIO2) (in all
cases, Ar represents an aryl group, such as phenyl). Some of the historically-important synthetic
methodologies used to prepare the first polyvalent iodine compounds are presented in Figure 1.
Cl aq. NaOH I
[Hartmann & Meyer, 1894]
Ag2O / H2O
OHFigure 1. Early examples of preparation of polyvalent iodine(III) and iodine(V) compounds.
The existence of iodine compounds, where the halogen formed more than one bond
seemed to contradict the octet rule of bonding, introduced by Lewis. At first, the bonding in
these and many other similar main-group-element compounds, in which the “standard” valence
number was exceeded, was conveniently described in terms of “promotion” of one or more p- or
even s-electrons (total number of n) to one or more vacant d-orbitals, followed by formation of
dn orbitals. However, in the early 1950s and 1960s, with the development of
theoretical methods (according to which, the “promotion” of an electron would require to high
energy, which made that process unlikely) as well as with the accumulation of structural data of
molecules containing polyvalent main group atoms, doubts began to be raised about the
participation of d-orbitals in the bond formation in such species. In fact, as early as 1951, it was
proposed that only p-orbitals of the halogen atoms participated in the bonding. Essentially,
delocalized, multicenter-multielectron, specifically, 3-center-4-electron (3c-4e), bonds were
suggested to be formed. This model was successfully employed to describe the bonding and
the geometry of compounds of the noble gases, such as XeF2 and XeF4, and many others. In
1969, in a seminal work, Musher introduced the name hypervalent (HV) bonds for those formed
using doubly-occupied with lone pairs p- (or sometimes even s-) orbitals. The formation of 3c-4e
HV bonds involving a main-group atom X having at least one lone pair and two atoms (or
groups), typically dubbed ligands, each possessing a single electron on a p-orbital (Li• and Lii•
is shown schematically in Figure 2.
) ϕ(Lii ϕ(X) )
Ψ1 = ϕ(Li
) + ϕ(X) + ϕ(Lii
Ψ3 = ϕ(Li
) - ϕ(X) + ϕ(Lii
Ψ2 = ϕ(Li
) - ϕ(Lii
L X i Lii
Figure 2. Formation of a linear fragment Li
-X-Lii via 3c-4e HV bonds between a central atom X
and ligands Li• and Lii•
, and energy diagram of the formed molecular orbitals.
The three p atomic orbitals combine to form three molecular orbitals: bonding (Ψ1 in
Figure 2), nonbonding (Ψ2, HOMO) and anibonding (Ψ3, LUMO). The four available bonding
electrons populate the first two molecular orbitals and the net result is that the two ligands
receive a partial negative charge, while the central atom becomes electrophilic due to partial
positive charge. As could be expected, the ligands in most stable HV compounds contain an
electronegative atom (F, Cl, O, N) directly bonded to the central HV atom.
HV bonds are weaker than typical (2c-2e) covalent bonds and as a result they can be
cleaved homolytically with the formation of the corresponding radicals (Figure 3, top). In
addition, HV bonds are dynamic in nature and ligand-exchange reactions with nucleophiles are
possible. A nucleophile (Nu-
) can substitute the ligands of attached to the central HV iodine
atom (either via associative or dissociation mechanism, which are analogues of SN2 and SN1
reactions). Moreover, because of the dynamic nature of HV bonds, if two HV iodine compounds
with different ligands are mixed, they exchange their ligands, and an equilibrium is established,
leading to the formation of a new, asymmetric HV iodine compound. The mentioned reactions
are presented in Figure 3.
+ 2 L
Nu Ar I
Ar I + 2 Nu
Nu - L
- Nu L
+ Ar I
Figure 3. Bond homolysis and ligand-exchange reactions involving HV iodine(III) compounds.
In this lab, a HV iodine compound, PhICl2 (Scheme 3, Ar = Ph, L = Cl) will be
synthesized and its ability to participate in ligand-exchange reactions with PhI(O2CCH3)2
(Scheme 3, Ar = Ph, L = CH3CO2) will be studied.
1. Explain why HV iodine compounds of the type ArIL2 contain the linear fragment L-I-L, and
why it is perpendicular to the I-C(Ar) bond.
2. Using the model of hypervalency, predict the structures (i.e., relative bond lengths and
approximate valence angles) of ICl3, BrF5, SeF2, and SeF4. How many bonds are “regular”
covalent and how many are hypervalent?
3. HV iodine compounds are often stored in dark bottles. Why? Do you expect these
compounds are sensitive to light with higher or lower wavelength than similar monovalent
iodine compounds and why? For example, compare C6H5ICl2 and C6H5I.
4. Do you expect that in the NMR spectra, the chemical shifts of the aromatic protons in C6H5I
and the carbon atoms will be at lower or higher field than those in C6H5ICl2 and why?
5. Propose a method that will allow you to determine the equilibrium constant of exchange (Kex
in Figure 3) in the reaction between C6H5ICl2 and C6H5I(O2CCH3)2 that is different than NMR
spectroscopy. What are the requirements for an experimental technique to be useful for
determination of the equilibrium constant.
6. Taking into account the structure of HV iodine compounds, e.g., C6H5I(O2CCH3)2, what
properties and applications would you predict they may have? Explain your answer.
7. Using the MO diagram shown in Figure 2, try to predict if a compound ArIL2 containing
ligands L with very electronegative atoms (e.g., F, O, Cl) or less electronegative atoms (e.g.,
Br, S, N) are likely to be more stable.
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1. Explain why HV iodine compounds of the type ArIL2 contain the linear fragment L-I-L, and why it is perpendicular to the I-C(Ar) bond. The HV iodine compounds of the type ArIL2 contain the linear fragment L-I-L, because L-I-L fragment can be described as delocalized, 3-center-4-electron bond. In that framework, the interaction occurs between full p-orbital of I (2 electrons) and two L· radicals (2*1=2 electrones) and their (most often p) orbitals. These three p orbitals combine to form 3 molecular orbitals (MOs), 1 bonding, 1 non-bonding and 1 anti-bonding. Since there are total 4 electrons involved, the bonding and non-bonding orbitals are occupied, giving in total ½ bond order for each I-L bond.
Since we explained that the same p-orbital is involved in interaction with both L ligands, it is clear why the fragment is linear (the p-otbital itself is linear). Since I-C(Ar) bond must be formed with the remaining p-orbitals (which are mutually perpendicular), this explains why L-I-L fragment is perpendicular to the I-C(Ar) bond...