Crystal Structure of [PPN+][Cr(CO)4NO-]:  Calculations and Comparisons of Cr(CO)4NO- with Mn(CO)5

The crystal structure of the deep red crystalline salt PPN+Cr(CO)4NO- has been determined and can be described as a distorted trigonal bipyramid. As expected, the NO group is equatorial, and the major distortion in Cr(CO)4NO- involves the axial CO groups, with an angle Cax−Cr−Cax of 159.08(17)°. These CO groups form this reduced angle by bending away from the NO group. This distorted structure is not primarily due to lattice effects, since high-level MO calculations show a similar geometry for the Cr(CO)4NO- part by itself. These MO calculations were also carried out on Mn(CO)5 -, and as expected this anion is quite fluxional. In contrast, we were unable to find any other low-energy geometry for Cr(CO)4NO-.