Several kinds of nitrosylpentaamminechromium (III) complexes [Cr(NO)(NH_3)_5]^<2+> (A) with different anions were prepared and widely characterized, and it was reported that the colors in the solid states of the compounds containing A's depended very much on the kinds of counter anions. None of the crystal structures, however, were determined, and the cause of the color change was unclear for a long time.
We have succeeded in the X-ray structural determination of five kinds of the complexes, and found very little dimensional differences of the cations. However, the color, shortest O(NO)-H(NH_3 in the adjacent cation) distance (Å), peak position of reflection spectra (nm), wave number of N-O stretching vibration (cm^<-1>) of the complex compounds change considerably : A(PF_6)_2 (red, 2.399, 579, 1693, 180), ACl_2 (red-orange, 2.749, 588, 1683), ACl(ClO_4) (brown, 3.127, 598, 1710), ACl(PF_6) (brown, 3.258, 603, 1713), and A(ClO_4)_2 (green, 3.595, 623, 1728). Comparison of the five crystal structures has led to the conclusion that the existence of the strong hydrogen bonds between the O (nitrosyl) and H (ammonia in adjacent complex cations) atoms gives red crystals of A(PF_6)_2, while the absence of hydrogen bonds results in the formation of green crystals of A(ClO_4)_2.
Optimized structures of the complex A at three different spin multiplicities, doublet (A2), quartet (A4), and sextet (A6), were obtained by ab initio calculation at B3LYP/6-31G^★(6-31+G for NO) level of theory. The complex was most stable at the lowest spin multiplicity (doublet), which is in harmony with the X-ray structure of A in A(ClO_4)_2 and with the previous magnetic measurement (μ_<eff> = 1.8 BM) of A(ClO_4)_2.
Optimized structures of the complexes, [Cr(NO)(CN)_5]^<3->, [Cr(CN)(NH_3)_5]^<2+>, [Cr(N_2)(NH_3)_5]^<3+>, [Cr(CO)(NH_3)_5]^<3+>, [Cr(O_2)(NH_3)_5]^<3+> were obtained, while only the X-ray structure of [Cr(NO)(CN)_5]^<3-> has been determined among them.