Spectroscopic and structural studies of chromate ions in zinc complexes with 2,2 '-bipyridine. Analysis of the lowest triplet states in the CrO42- entity

Two novel complexes of Zn(II) chromate with 2,2'-bipyridine have been synth esised: [Zn(bpy)(3)]CrO4.7.5H(2)O (1) and catena-(mu -CrO4-O,O')[Zn(bpy)(H2 O)(2)]. 2H(2)O (2). Complex 1 has been characterised by a structural method . The [Zn(bpy)(3)]CrO4.7.5H(2)O crystals have a monoclinic symmetry with sp ace group C2/c and eight chemical units. The chromate ion is not coordinate d to the zinc(II) ion. The O(3) and O(4) atoms of CrO42- and O(8) of the wa ter molecule statistically occupy their position with k = 0.5, which means that the chromate ions execute reorientational motion between two equilibri um arrangements with equal probability. 4 K electronic spectra (1) revealed the vibrational fine structure in v(3)(F-2) = 820 cm(-1) for the spin-forb idden (1)A(1) --> T-3(1) transition. The pure electronic 0-0 transition in (1)A(1) --> T-1(1) was found at 20 270 cm(-1). In complex 2 the broad low i ntensity band at ca. 16 800 cm(-1) has been assigned to a forbidden Zn-O-Cr transition in the bridge. (C) 2001 Elsevier Science Ltd. All rights reserv ed.