Syntheses and crystal structures of three novel oxalate coordination compounds: Rb 2 Co(C 2 O 4 ) 2 ·4H 2 O, Rb 2 CoCl 2 (C 2 O 4 ) and K 2 Li 2 Cu(C 2 O 4 ) 3 ·2H 2 O.

Single crystals of three novel transition-metal oxalates, dirubidium di-aqua-dioxalatocobalt(II) dihydrate or dirubidium cobalt(II) bis-(oxalate) tetra-hydrate, Rb 2 [Co(C 2 O 4 ) 2 (H 2 O) 2 ]·2H 2 O, (I), catena -poly[dirubidium [[di-chlorido-cobalt(II)]-μ-oxalato]] or dirubidium cobalt(II) dichloride oxalate, {Rb 2 [CoCl 2 (C 2 O 4 )]} n , (II), and poly[dipotassium [tri-μ-oxalato-copper(II)dilithium] dihydrate] or dipotassium dilithium copper(II) tris-(oxalate) dihydrate, {K 2 [Li 2 Cu(C 2 O 4 ) 3 ]·2H 2 O} n , (III), have been grown under hydro-thermal conditions and their crystal structures determined using single-crystal X-ray diffraction. The structure of (I) exhibits isolated octa-hedral [Co(C 2 O 4 ) 2 (H 2 O) 2 ] units, whereas (II) consists of trans chains of Co 2+ ions bridged by bidentate oxalato ligands and (III) displays a novel tri-periodic network of Li + and Cu 2+ ions linked by oxalato bridging ligands.