Crystal structure of [PPN+][Cr(CO)(4)NO-]: Calculations and comparisons ofCr(CO)(4)NO- with Mn(CO)(5)(-)

The crystal structure of the deep red crystalline salt PPN+Cr(CO)(4)NO- 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)(4 )NO- involves the axial CO groups, with an angle C-ax-C-r-C-ax of 159.08(17 )degrees. 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 shoal a similar geometry for the Cr(CO)4N O- 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 una ble to find any other low-energy geometry for Cr(CO)(4)NO-.