Electronic properties and crystal structure of (2,2′-bipyridyl)-catena-μ-(oxalato-O¹O²: O^1′O^2′)-copper(II) dihydrate and aqua(2,2′-bipyridyl)-(oxalato-O¹O²)copper(II) dihydrate

The crystal structure of the title compounds [Cu(bipy)(C₂O₄)]·2H₂O (1) and [Cu(bipy)(C₂O₄)(OH₂)]·2H₂O (2) have been determined by X-ray analysis. Compound (1) crystallises in the triclinic space group P1 with a = 9.673(3), b = 8.940(3), c = 9.103(3) Å, α = 105.718(3), β = 110.347(3), γ = 97.539(3)°, and Z = 2. The six-co-ordinate CuN₂O₂O′₂ chromophore of (1) involves an elongated rhombic octahedral stereochemistry involving a symmetrically co-ordinated bipy ligand (mean Cu-N 2.007 Å) and unsymmetrically co-ordinated bridging oxalate groups (mean Cu-O 1.988 and 2.320 Å). Compound (2) crystallises in the triclinic space group P1̄ with a = 10.565(3), b = 7.246(3), c = 10.806(3) Å, α = 102.467(3), β = 62.119(3), γ = 98.134(3)°, and Z = 2. The CuN₂O₂O′ chromophore of (2) is basically square pyramidal with a symmetrically co-ordinated bipy ligand (mean Cu-N 1.989 Å), and a symmetrically co-ordinated oxalate group (mean Cu-O 1.953 Å) in the plane of the square pyramid, and a water molecule at 2.341 Å out of the plane. The electronic reflectance spectrum of (1) involves main band at 14 500 cm^-1 with a resolved broad band at 9 300 cm^-1, while that of (2) involves a single broad band at 15 600 cm^-1, a difference that is consistent with the structures and suggests an 'electronic criterion of stereochemistry' to distinguish these two structures.