  We investigate the spectroscopic magnetic excitations in molecular magnets
with complex intermediate structure among the magnetic ions. Our approach
consists in introducing a modified spin Hamiltonian that allows for discrete
coupling parameters accounting for all energetically favorable spatial
distributions of the valence electrons along the exchange bridges connecting
the constituent magnetic ions. We discuss the physical relevance of the
constructed Hamiltonian and derive its eigenvalues. The model is applied to
explore the magnetic excitations of the tetrameric molecular magnet
Ni$_4$Mo$_{12}$. Our results are in a very good agreement with the available
experimental data. We show that the experimental magnetic excitations in the
named tetramer can be traced back to the specific geometry and complex chemical
structure of the exchange bridges leading to the splitting and broadness of the
peaks centered about 0.5 meV and 1.7 meV.
