90 INSTRUMENTAL METHODS
displayed in the form of an ellipsoid of vibration, commonly so that the vibrat-
ing atom has a 50% chance of being within the ellipsoid.
This is illustrated in Figure 3.8 for a hemoglobin model compound, featur-
ing one of a large group of so - called “ picket fence porphyrin ” ligand systems,
synthesized in the James P. Collman laboratory in the late 1970s. More detail
on hemoglobin model compounds will be found in Sections 7.2.4 through 7.2.7 ;
here we will concentrate on the X - ray diffraction study of this important
hemoglobin model. The researchers began with crystals of (2 - methylimid-
azole) -meso - tetrakis( α , α , α , α - o - pivalamidophenyl)porphyrinatoiron(II) -
ethanol, [(Fe(T piv PP)(2 - MeIm)] - EtOH (see Figures 7.9B and 7.10 for
structures). They exposed the [(Fe(T piv PP)(2 - MeIm)] – EtOH crystals to dioxy-
gen, and then they determined the structure of the starting complex as well as
the resulting dioxygen complex using X - ray diffraction.^15 The deoxy starting
complex was found to have a C 2 molecular symmetry with space group
CCc 26 h− 2 / ,Z = 4 (four molecules in the unit cell), and a unit cell with the
following dimensions: a = 18.871(11) Å , b = 19.425(13) Å , c = 18.434(11) Å ,
β = 91.48 ° , and volume, V = 6755.0 Å^3. The oxy complex was nearly isomor-
phous in space group CCc 26 h− 2 / ,Z = 4, and had very similar cell dimensions.
The structures were refi ned by a full - matrix, least - squares methods, using 4176
and 5183 refl ections for the deoxy and oxy complexes, respectively. Selected
bond distances and angles for the oxy complex [(Fe(O 2 )(T piv PP)(2 - MeIm)] –
EtOH are: Fe – N porphyrin = 1.997(4), 1.995(4) Å ; Fe – N Im = 2.107(4) Å ; Fe –
O = 1.898(7) Å ; O – O = 1.22(2) Å ; Fe – O – O angle = 129(1) °. The iron ion is
displaced 0.399 Å from the plane of the porphinato nitrogen atoms toward the
imidazole ligand in the deoxy complex, but only 0.086 Å in the oxy complex.
Figure 3.8 Ellipsoids of vibration for a small molecule — an ORTEP view. Stereodia-
gram of [(Fe(O 2 )(T piv PP)(1 - MeIm)] looking down the C 2 twofold axis of symmetry.
(Reprinted with permission from Figure 6 of reference 16. Copyright 1978, American
Chemical Society.)