PART. 1: The crystals of Ni(histidine)₂.H₂0 are composed of two kinds
of complex: Ni(D histidine)₂ and Ni(L histidine)₂ related to each
other by glide planes. Each histidine ligand is tridentate, and
is co-ordinated to the nickel atom by the imidazole nitrogen(N₂)
the "peptide" nitrogen (N₁) and a carboxyl oxygen(O₁). The bonds
to Ni do not differ significantly in length, and agree with those
quoted in literature. The co- ordination of Ni is very nearly
regular octahedral. The two histidine ligands attached to Ni,
are related by a two -fold axis passing through Ni, the two bulky
imidazole rings being trans to one another, with O₁ and O'₁ cis
and also N₁ and N'₁ cis. Comparison with the structures of the two
histidine complexes [3,4] and histidine hydrochloride [14]
shows a significant difference in histidine conformation, brought
about by rotation about the C₃ -C₄ bond. Also the conformation
along the N₁ -C₂ bond is not staggered as in other determinations,
and this is probably due to the stronger binding of O₁ and the
fewer degrees of freedom in the nickel complex. The "peptide
group" is not planar, the nitrogen deviating by about .3 °A from
the plane of the other atoms, as has been found elsewhere [4,14].
The crystal, which is very stable, is held together by an
intricate system of hydrogen bonds, with Van der Waals' contact
between parallel imidazole rings. The single water molecule does
not hydrogen-bond in such a way as to help in binding molecules
together, but seems to fill a vacant space in the crystal.
PART.2: DETERMINATION of the STRUCTURE of BIS(DIMETHYLDITHIOCARBAMATO)
PYRIDINE ZINC.
The doubt about the light atom positions in the Fourier
refinement of the b -axis projection, showed that the data was not
good enough to be able to locate them with sufficient accuracy.
The analysis could have been carried into three dimensions, but
there would still be inaccuracies in the light atom parameters due
to the weakness or absence of the high order reflections. So
attempts were made to recrystallise the complex. It was noticed
at once that on recrystallising from a large excess of benzene,
fine needles were obtained that gave good X -Ray photographs. The
analysis of the b -axis projection was therefore abandoned.
Few conclusions of any interest can be drawn from this analysis
itself. However comparison with the projection down the molecular
two -fold axis of the complex (see Fig. 18) reveals some interesting
points: -
(i) The b -axis Patterson and Fourier (Figs. 9 and 10) show
quite a good agreement with the final structure, as determined in
the crystals with benzene of crystallisation. The heavy atom
positions agree well, but some of the light atom peaks, especially
C7 and C8, are slightly off their expected positions. But there
can be no doubt that in these crystals the molecular two -fold axis
lies parallel or very nearly parallel to the b -axis. There is
only one significant difference - the orientation of the pyridine
ring. Both the Patterson and the Fourier indicate that the
pyridine plane passes between Si and S2, an orientation about 90°
from that in the crystals with benzene of crystallisation.
(ii) The final analysis shows that the complex can probably
possess exact two -fold symmetry in solution, although it does not
in the crystals with benzene of crystallisation. It is very
likely therefore that it will crystallise on a crystallographic
two -fold axis whenever possible. This isstrong evidence for C2 /c
as opposed to Cc. However it does not constitute proof. There
is still the possibility, however unlikely, that the true space
group is Cc. The two space groups are closely related. If
objects of two -fold symmetry are placed in space group Cc with
their axes parallel to b, the symmetry is transformed to that of
C2 /c. It is clear that if the objects are not exactly symmetric,
the symmetry cannot be exactly C 2 /c, although it must closely
resemble it. Undoubtedly the symmetry of these crystals is very
near to C 2 /c, but this cannot be taken as proof that the complex
possesses an accurate two-fold axis.
There does not seem to be any method capable of distinguishing
the two space groups in a case like this. The statistical test of
intensities would certainly indicate a centrosymmetric structure.
(iii) Packing.
The structure consists of sheets of molecules perpendicular to
the c -axis. One sheet is related to the next by the c glide,
but due to lack of knowledge of the y co- ordinates, the interactions of one sheet with the next cannot be described in detail.
However it can be said that their methyl groups are in contact,
and one sheet "lies" on the methyl groups of its neighbour.
The packing within a sheet is of course face centred. The
distance from C3 of pyridine to Zn along y is 4.1 5, and the
pyridines therefore fit into the "pocket" of sulphur atoms of the
molecule next along the y axis. Each pyridine probably has two
contacts with methyl groups of C face -centred neighbours, one on
either side. There are no pyridine -pyridine contacts as in the
crystals with the benzene of crystallisation. This packing
arrangement is consistent with the orientation of the pyridine
ring.
PART3: DETERMINATION OF THE STRUCTURE OF BIS(DIMETHYLDITHIOCARBAMATO)
PYRIpINE ZINC WITH BENZENE OF CRYSTALLISATION
The complex (PZDC) is a 5- coordinate zinc complex with a
distorted trigonal bipyramidal configuration. The two axial
Zn -S bonds average 2.60 °A, with the two equatorial Zn -S bonds,
2.33 °A and Zn -N₁ 2.08 °A The equatorial bonds are of normal
length, but the two axial bonds are considerably longer than usual.
This is probably due to a combination of electron pair repulsions,
and a weakening of the axial Zn -S bonds due to the coordination of
pyridine. The mean N-C bond length in the dithiocarbamate groups
is 1.338 °A , which is very similar to the lengths found in all other
dithiocarbamate complexes, and indicates about 84% double bond
character. This agrees with the experiments of Chatt et al [69]
and is further evidence against the theories of Kerk et al [68].
Both the Zn -S and the N-C bond lengths show that the electron shift
scheme for amine coordination proposed by Higgins and Saville, must
be revised. The coordination of an amine appears to affect only
two of the four Zn -S bonds, and has no effect on the N -C bond.
In agreement with other determinations, the dimethyldithiocarbamate groups are both planar and the bond lengths and angles are
all normal. Zn, however, lies slightly out of the plane of both
ligands, and its position is different relative to each. The
Me N Me angle is slightly less than 120° as has been found also by
Vaciago et al [63]. This is probably due to repulsion of the
methyl groups by the sulphurs.
There is an approximate two-fold axis through Zn, N₁ and C₃,
but the axis of the pyridine ring deviates by 4° from the axis of
the rest of the molecule. This is caused by Van der Waal's
contacts in the crystal. The difference in the deviations of Zn
from the dithiocarbamate planes may be connected with this. It
is very probable that the complex can assume accurate two -fold
symmetry in solution. The orientation of pyridine is obviously
largely dependent on intermolecular forces, since it is different
in the crystals without benzene of crystallisation.
The crystals with benzene of crystallisation, consist of two
types of complex, D and L. The packing is rather irregular, and
there are some large empty spaces especially between benzene
molecules along the y-axis.
