DFT calculation and X-ray structure of nitrosyl pentaammine chromium complex

The optimized structures of nitrosyl pentaammine chromium complex [Cr(NO)(NH3)5]2+ (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 A was most stable at the lowest spin multiplicity (doublet), which is in harmony with the previous magnetic measurement (μeff=1.8 BM) of the perchlorate of A, [Cr(NO)(NH3)5](ClO4)2 (A′). The X-ray structure of A′ was determined: Cr–N(NO), 1.677(6) Å; N–O, 1.180(7) Å; Cr–N(NH3,trans), 2.140(5) Å; Cr–N(NH3,cis), 2.080(6)–2.096(6) Å; Cr–N–O, 179.9(6)°. The optimized structure (A2) is in good agreement with the experimental one. The present result shows that the covalent type π bondings between Cr and N(NO), which is ascribed to the overlapping between two π∗ orbitals of NO and two π orbitals of Cr, cause the extra stabilization of A2.