  The magnetic properties of metal-organic complexes are strongly influenced by
conformational changes in the ligand. The flexibility of
Fe-tetra-pyridyl-porphyrin molecules leads to different adsorption
configurations on a Au(111) surface. By combining low-temperature scanning
tunneling spectroscopy and atomic force microscopy, we resolve a correlation of
the molecular configuration with different spin states and magnitudes of
magnetic anisotropy. When the macrocycle exhibits a laterally-undistorted
saddle shape, the molecules lie in a S=1 state with axial anisotropy arising
from a square-planar ligand field. If the symmetry in the molecular ligand
field is reduced by a lateral distortion of the molecule, we find a finite
contribution of transverse anisotropy. Some of the distorted molecules lie in a
S=2 state, again exhibiting substantial transverse anisotropy.
