COMBINED X-RAY CRYSTALLOGRAPHIC, SINGLE-CRYSTAL EPR, AND THEORETICAL-STUDY OF METAL-CENTERED RADICALS OF THE TYPE (ETA-5-C5R5CR(CO)2L) (R = H, ME L = CO, TERTIARY PHOSPHINE)

The X-ray crystal structures of the compounds eta-5-C5Me5Mn(CO)3, {eta-5-C5Me5Cr(CO)2(PMe3)}, and eta-5-C5Me5Mn(CO)2(PMe3), the latter two new, have been determined: eta-5-C5Me5Mn(CO)3 crystallizes in the centric space group P2(1)/m, eta-5-C5Me5Mn(CO)2(PMe3) in Pbca, and {eta-5-C5Me5Cr(CO)2(PMe3)} in Pmnb. All three compounds assume essentially ''piano-stool-like'' structures, the manganese compounds with OC-Mn-CO and OC-Mn-P bond angles of congruent-to 92-degrees, normal for this type of 18-electron compound. In contrast, the 17-electron chromium compound exhibits a closed-in OC-Cr-CO bond angle of only 79.9 (1)-degrees, comparable with that of the only other such compound previously reported, {eta-5-C5H5Cr(CO)2(PPh3)} (80.9 (1)-degrees. Single crystals containing about 1% of {eta-5-C5Me5Cr(CO)3} and {eta-5-C5Me5Cr(CO)2(PMe3)} doped into their manganese analogues have been studied by EPR spectroscopy at 20 K, and the g and P-31 hyperfine tensors have been assembled in the crystal-axis systems of the hosts. Although interpretation of the data for {eta-5-C5Me5Cr(CO)3} is not unambiguous, 2A'' ground states seem likely for both it and {eta-5-C5Me5Cr(CO)2(PMe3)}, as was observed previously for {eta-5-C5H5Cr(CO)2(PPh3)}. Complementing the above, LACO-HFS calculations for {eta-5-C5H5Cr(CO)3} and {eta-5-C5H5Cr(CO)2(PH3)} have also been carried out, the nature and geometry-optimized structures of the ground states being determined. While {eta-5-C5H5Cr(CO)3} is predicted to assume a 2A' ground state, the first excited state, of 2A'' character, lies only some 7 kJ mol-1 higher; thus both states should be populated in the gas phase, and the relative ordering in condensed media may well be sensitive to the environment. Interestingly, {eta-5-C5H5Cr(CO)2(PH3)} is predicted to assume a 2A'' ground state with an optimized geometry in which the OC-Cr-CO angle is congruent-to-80-degrees, in agreement with the EPR data and the X-ray crystal structures of both {eta-5-C5Me5Cr(CO)2(PMe3)} and {eta-5-C5H5Cr(CO)2(PPh3)}.