Preparation, structure, and bonding of two cubane-like iron-nitrosyl complexes, Fe4(NO)4(.mu.3-S)4 and Fe4(NO)4(.mu.3-S2)(.mu.3-NC(CH3)3)2. Stereochemical consequences of bridging ligand substitution on a completely bonding tetrametal cluster unit and of different terminal ligands on cubane-like FeS