Transition metals are generally not modeled well with additive force fields, and those primarily developed for use with biomolecules will certainly not support such species. CGenFF is for drug-like molecules, so you can't generally get a topology for anything containing a metal.

Hi Akhil,

You can also model your system with MCPB.py available in Ambertools17.

Article MCPB.py: A Python Based Metal Center Parameter Builder

http://ambermd.org/tutorials/advanced/tutorial20/mcpbpy.htm