Investigation of Structural Characteristics of Bis(beta-diketonato)copper(II) Complexes Containing Alkoxy or Aryloxy Side Chains: X-Ray Structures of 1,3-Bis(4-butoxyphenyl)-propane-1,3-dione, Bis[1,3-bis(4-butoxyphenyl)propane-1,3-dionato-kappaO,sigmaO']copper(II) and Bis[1,3-bis(4-phenoxyphenyl)propane-1,3-dionato-kappaO,kappaO']copper(II)