Linear trinuclear transition-metal compounds containing 3,5-diethyl-1,2,4-triazole and fluoride as bridging ligands. X-ray structure of bis[(.mu.-fluoro)bis(.mu.-3,5-diethyl-1,2,4-triazole-N1,N2)bis(thiocyanato-N)(3,5-diethyl-1,2,4-triazole-N1)cobalt(II)-F,N1,N1']cobalt(II) dihydrate