题目内容：

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Table 1 compares some important distances in the
molecules.
Connector Size. The results permit a fairly accurate
extrapolation to estimate connector size. The expected
radius of the pyridine- and bipyridine-containing connectors
is about 14.3 and 18.5 Å, respectively. However,
because of the location of the binding sites, the effective
separation of a metal site from the center of the molecule
is larger in the pyridine-terminated than in the bipyridine-
terminated structure and is 16.9 and 15.0 Å, respectively.
The result is important for the estimation of distances
in potential molecular structures built with the connectors.
For example, the bipyridyl and bipyrimidine units
complex well to mercury atoms15 and we are hoping to
exploit such complexation in molecular construction.
The X-ray structures of the compounds also provide
additional data concerning the use of bicyclo[1.1.1]-
pentane modules in molecular rods. The presently observed
distances between bridgehead carbon atoms (1.87-
1.88 Å) are somewhat longer than the average for all such
distances in the Cambridge database (1.81 Å).14 Of
course, this carbon-carbon bond distance is long when
compared to the “parent module”, [1.1.1]propellane,
where it is only 1.587 Å.53