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ionic salt [145] for tetramethylenetetramine, highlights the obvious diffi-
culty in predicting which mode will be favored for a given donor-acceptor
pair.
Oxygen complexes with dihalogens are relatively rare, with only eight
complexes known, and none of these involves interhalogens. Diiodine and
dibromine complexes are equally represented with four and three known ex-
amples of each, respectively, and one dichlorine complex is known. It should
be pointed out that this complex, dioxane dichlorine [41], is the only halo-
gen bonded complex of dichlorine that has been structurally characterized.
As opposed to those observed for nitrogen, all of the oxygen complexes ex-
cept one crystallize with bridging dihalogens (mode
B
). The sole exception is
a relatively complex manganese formate salt, in which an I
2
molecule inter-
acts as a simple adduct [146].
2.3
Phosphorus and Sulfur Electron Donors
True halogen bonded complexes with phosphorus as the electron donor atom
are even more rare than those with oxygen, and all of these involve diiodine as
the electron acceptor. Three examples of simple adducts and one with a bridg-
ing diiodine were found. All other phosphorus halogen complexes are better
described as salts with ion pairs closely associated through X
Xinterac-
tions. One bromo- (JOMSEJ) and two iodo salts (SORPUK and ZEKMOR) are
related to simple adduct (mode
A
, Eq. 1) complexes, in which the P
···
Xin-
teraction has strengthened to a covalent bond, while the X-X bond has weak-
ened to the point of being an interaction. Likewise, one bromo- (SUHJIO) and
five iodo- (LOBKOC, LOBKUI, LOBLAP, FILTEZ, and FILTID) salts with tri-
halide anions can be considered as coming from extended adducts (mode
AA
,
Eq. 2).
···
Equation 1
Equation 2
With 112 structurally characterized complexes, sulfur-based electrons are
by far the most commonly encountered. This is attributed to its importance
in antithyroid drugs, as described above. The majority of these complexes
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