AN X-RAY DETERMINATION OF THE ELECTRON-DISTRIBUTION IN CRYSTALS OF HEXAPYRIDINE-N-OXIDE COBALT(II) PERCHLORATE AND THE ELECTRONIC-STRUCTURE OF THE CO2+ ION

The valence electron density distribution in hexapyridine-N-oxide cobalt(II) perchlorate, Co(C5H5NO)(6)(ClO4)(2), has been investigated experimentally using high resolution single crystal X-ray diffraction data collected at 78(1) K with Mo K alpha radiation, together with the results from the already reported neutron structure determination. The crystal data at 78 K are a=12.317(1) and c=18.826(1) Angstrom; space group R (3) over bar, Z=3. Deformation electronic density maps have been calculated from the X-ray data, using parameters from the refinement of the high order data, sin theta/lambda > 0.75 Angstrom(-1), the parameters obtained from the refinement of the neutron data and from structure factors obtained from a multipole deformation refinement. Conventional spherical atom refinement on 4030 reflections (F-o greater than or equal to 3 sigma F-o) gives R=0.037 and R(w)=0.038, while the multipole refinement on 3627 reflections gives R=0.028 and R(w)=0.024. A pronounced aspherical electron distribution has been found for the cobalt atom in the deformation maps which is consistent with a significant trigonal distortion of the distribution expected for an octahedral ligand field, viz. t(2g)(5.0) and e(g)(2.0). The populations obtained from the multipole refinement confirm this and give apparent orbital occupancies of 3.7(1), 1.2(1) and 2.4(1) for the e(g)(t(2g)(+/-)), a(g)(t(2g)(0)) and e'(g)(e(g)) orbitals respectively, the charge on the cobalt ion being determined as +1.74(4). There is a large e(g)-e'(g) mixing coefficient which leads to the conclusion that there is a significant departure from the t(2g) octahedral character of the ground state orbitals. The results are consistent with a (4)A(g) ground state for the Co2+ ion, which is in accord with the various earlier magnetic, spectroscopic and heat capacity studies.