A comparative study of the use of triethylammonium salts of the [Fe2(CO)6(μ-CO)(μ-SR)]- anion in the synthesis of iron-gold clusters. Crystal structures of [Fe2(CO)6(μ-CO)(μ-SiPr)(μ-AuPPh3)] and [Fe2(CO)5(PPh3)(μ-SEt)2]