Structural and spectroscopic study of cobalt(III) ternary complexes
Abstract The trans-diazidobis(ethylenediamine)cobalt(III)perchlorate complex with the formula [Co(en)2(N3)2]·ClO4 (1) (en = ethylenediamine) crystallises in the centrosymmetric space group C2/c of the monoclinic system. The almost ideal octahedral geometry of the cobalt atom is created by six nitrogen atoms of two ethylenediamine molecules and two azide ions, which form the trans-[CoN4N2′] chromophore. The crystal of 1 is built up of [Co(en)2(N3)2]+ cations and non-coordinated ClO4− anions. The lattice perchlorate ions adopt an atypical C2v symmetry. A hydrogen bonded system is created by the O1 and O2 perchlorate atoms and atoms of the NH2 groups, which generate the 3D-structure. The vibrational and electronic spectra of 1, [Co(en)2CO3]·ClO4 (2) and [Co(bpy)2CO3]·NO3·5H2O (3) (2,2′bpy or bpy = 2,2′bipyridine) have been also studied. The Co(III) ion in all the complexes is in the typical low-spin state. Bands centered at ca. 18 030 and 24 120 cm−1 are characteristic for the trans-[CoN4N2′] chromophore of 1 and are assigned to the spin-allowed 1A1g → 1T1g and 1A1g → 1T2g transitions. The spectra of the cis-[CoN4O2] chromophore (2 and 3) exhibit an absorption at ca. 19 500 cm−1, which arises from the spin-allowed 1A1g → 1Eg + 1A2g transitions in a D4h symmetry. The descending spin-allowed 1A1g → 1B2g and 1A1g → 1Eg transitions are correlated with bands at ca. 25 500 and 29 000 cm−1, respectively.