¹H NMR RESEARCH İNTO DİAMAGNETİC 1,2,4, 1′ ,2′, 4′ -HEXAMETHYLFERROCENE, PARAMAGNETİC HEXAFLUOROPHOSPHATE OF 1,2,4, 1′ ,2′ ,4′ -HEXA­METHYL­FERRİCİNİUM AND ELECTRON EXCHANGE BETWEEN THEM