The structures of the sterically protected "pocket" porphyrin H-2(alpha-PocPivP) (1), and the "capped" porphyrins H-2(C-3-Cap) (2), Fe(C-3-Cap)(CO)(1-MeIm) (3), H-2(C-4-Cap) (4), and Fe(C(4)Cap)(Cl) (5) have been determined by single-crystal X-ray diffraction methods. Compounds 1-4 each pack with one independent porphyrin unit and solvate molecules in the unit cell. Compound 5 packs with two crystallographically independent porphyrins and solvate molecules in the unit cell. The structure of 1 is the first of a pocket porphyrin in which there are no ligands bound inside the protected region. This structure reveals that the main form of distortion that occurs when a ligand binds inside the protected region is the lateral movement of the benzene cap relative to a position above the centroid of the porphyrin. In 1 this lateral displacement is 1.86 Angstrom whereas this displacement is 3.30 Angstrom in Fe(beta-PocPivP)(CO)(1,2-Me(2)Im). Comparison of the free-base structure 2 with the Fe-II carbonyl 3, where the CO ligand is bound under the cap and the 1-MeIm ligand is bound opposite the cap, reveals that there is little lateral distortion in the C-3-Cap system, but there is significant vertical expansion of the cap upon coligation to the Fe center. The distance of the cap centroid from the mean porphyrin plane increases 2.37 Angstrom to accommodate the CO ligand, from 3.49 Angstrom in 2 to 5.86 Angstrom in 3. The Fe-C-O angle in 3 is 178.0(13)degrees. In the structures of the C-4-Cap system 4 and 5 there is sufficient space for the binding of small ligands, such as CO and O-2, as well as larger ligands. Compound 4 crystallizes with a CHCl3 solvate molecule under the cap. The distances from cap centroid to porphyrin plane in the C-4-Cap structures are 7.28, 7.12, and 7.66 Angstrom for 4, 5A, and 5B, respectively. This is significantly greater than the distance of 5.6 Angstrom in Fe(C-2-Cap)(CO)(1-MeIm) and 5.86 Angstrom in 3. The relation between these structural changes on ligation and the binding properties of these systems for CO and O-2 is explored.