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62
RADIOCHEMISTRY, STABLE ISOTOPES,
NUCLEAR ANALYTICAL METHODS, GENERAL CHEMISTRY
pounds through entering (or not entering) the structure.
In compounds 1 and 3, the counter anions
build bridges within the same polymeric chain,
Fig.4. The structure of [Pb(trop) 2
] 2
(4). Selected bond lengths
[Å] and angles [ o ]: Pb1-O1 2.449(19), Pb1-O2
2.316(18), Pb1-O12 2.285(17), Pb1-O11 2.307(18),
O1-C1 1.222(28), O2-C2 1.284(31), O11-C11 1.321(21),
O12-C12 1.275(28), Pb1-O11’ 2.912, O1-Pb1-O2
65.61(59), O11-Pb1-O12 67.95(54).
whereas in compound 2 they link two adjacent polymeric
chains forming a three-dimensional lattice.
A comparison of the respective Pb-O bond distances
shows that perchlorate anions interact with
metal atoms more weakly than the triflate and nitrate
groups.
The formation of these four structures probably
depends on the pH of the solution. Because
of hydrolysis of the lead(II) salts, the pH of aqueous
solutions of salts originating from strong acids
(Pb(CF 3 SO 3 ) 2 , Pb(ClO 4 ) 2 and Pb(NO 3 ) 2 ) was lower
(pH~3 or less) than that (pH>5) of Pb(CH 3 COO) 2
– a salt of weak acid. In more acidic solutions, what
is in the case of the first three salts, the formation
of polymeric compounds followed the addition of
tropolone. At higher pH value, as for the lead(II)
acetate, we observed the precipitation of bis(tropolonato)lead(II).
Contrary to complexes 1 and 4, which have only
one kind of lead center, complexes 2 and 3 comprise
two coordinatively different types of lead
atoms. In all the complexes studied, tropolone chelates
the lead(II) ion in an anisobidentate manner,
with one shorter and one longer Pb-O bond. These
Pb-O bond lengths are in the range 2.28-2.45 Å.
The C-O bond distances in the studied structures
are intermediate between the C=O (1.26 Å)
and C-O (1.33 Å) bond lengths, which are observed
for the free tropolone molecule [14]. Only in the
purely chelating tropolonato ligand in 4, one of
the C-O distances (1.22 Å) is surprisingly shorter
than the C=O bond.
In the studied structures, the bridging Pb-O (trop)
distances vary in the range 2.44-2.91 Å. For compounds
4 and 1, we found one and two kinds of
such bridges, respectively. The largest variety of
side interactions between lead and tropolone appears
in polymers 2 and 3, where we have four
and three different bridging Pb-O (trop) contacts,
respectively, including the shortest one of 2.44 Å
for compound 2. Such an amazingly short bridge
is comparable with some chelating Pb-O (trop)
bonds, 2.455(12) Å (in 2) and 2.449(19) Å (in 4).
The studied complexes demonstrate various total
coordination numbers of lead(II) ions: from
5 in [Pb(trop) 2
] 2
, 7 in [Pb(trop)(CF 3
SO 3
)(H 2
O)] n
and [Pb 2 (trop) 2 (NO 3 ) 2 (CH 3 OH)] n to 8 in
[Pb 3 (trop) 4 (ClO 4 ) 2 ] n .
The nonspherical distribution of ligands surrounding
the lead(II) ion (hemidirected geometry
[15]) in the structures 1, 3 and 4 results from the
presence of the stereochemically active 6s 2 lone
electron pair. In compound 2, we can consider a
hemidirected geometry if we take into account only
tropolonato ligands. The gap observed in this complex
is filled, apart from the lone electron pair, with
the weakly bonding ClO 4– ions. The 6s 2 lone electron
pair on the lead(II) ions is stereochemically
active in all the complexes studied. The active lone
electron pair is common in lead(II) complexes with
coordination number up to 8, and does not occur
for higher coordination numbers [15].
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