  A method for generating angular forces around $\sigma$-bonded transition
metal ions is generalized to treat $\pi$-bonded configurations. The theoretical
approach is based on an analysis of a ligand-field Hamiltonian based on the
moments of the electron state distribution. The functional forms that are
obtained involve a modification of the usual expression of the binding energy
as a sum of ligand-ligand interactions, which however requires very little
increased in CPU time. The angular interactions have simple forms involving sin
and cos functions, whose relative weights depend on whether the ligands are
$\sigma$- or $\pi$-bonded. They describe the ligand-field stabilization energy
to an accuracy of about 10%. The resulting force field is used to model the
structure of small clusters, including fragments of the copper blue protein
structure. Large deviations from the typical square copper coordination are
found when $\pi$-bonded ligands are present.
