  We present a systematic investigation of molecule-metal interactions for
transition-metal phthalocyanines (TMPc, with TM = Fe, Co, Ni, Cu) adsorbed on
Ag(100). Scanning tunneling spectroscopy and density functional theory provide
insight into the charge transfer and hybridization mechanisms of TMPc as a
function of increasing occupancy of the 3d metal states. We show that all four
TMPc receive approximately one electron from the substrate. Charge transfer
occurs from the substrate to the molecules, inducing a charge reorganization in
FePc and CoPc, while adding one electron to ligand \pi-orbitals in NiPc and
CuPc. This has opposite consequences on the molecular magnetic moment: in FePc
and CoPc the interaction with the substrate tends to reduce the TM spin,
whereas in NiPc and CuPc an additional spin is induced on the aromatic Pc
ligand, leaving the TM spin unperturbed. In CuPc, the presence of both TM and
ligand spins leads to a triplet ground state arising from intramolecular
exchange coupling between d and \pi electrons. In FePc and CoPc the magnetic
moment of C and N atoms is antiparallel to that of the TM. The different
character and symmetry of the frontier orbitals in the TMPc series leads to
varying degrees of hybridization and correlation effects, ranging from the
mixed-valence (FePc, CoPc) to the Kondo regime (NiPc, CuPc). Coherent coupling
between Kondo and inelastic excitations induces finite-bias Kondo resonances
involving vibrational transitions in both NiPc and CuPc and triplet-singlet
transitions in CuPc.

Materials Science