I know of several general molecular mechanics methods suitable for metals, including Dreiding and UFF.
I know it's difficult to get many things right for such classical treatment of inorganic / organometallic molecules, because oxidation state, spin state and other factors can greatly influence geometries and energetics.
But I'm surprised that so few attempts have been made to improve on these methods. Are there no other methods published since 1992? I'm looking for force fields that cover a substantial part of the periodic table (or at least most metal-ligand interactions) and are completely published - proprietary methods don't count.