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

The valence electron density distribution in hexapyridine-N-oxide coba lt(II) perchlorate, Co(C5H5NO)(6)(ClO4)(2), has been investigated expe rimentally using high resolution single crystal X-ray diffraction data collected at 78(1) K with Mo K alpha radiation, together with the res ults from the already reported neutron structure determination. The cr ystal data at 78 K are a=12.317(1) and c=18.826(1) Angstrom; space gro up R (3) over bar, Z=3. Deformation electronic density maps have been calculated from the X-ray data, using parameters from the refinement o f the high order data, sin theta/lambda > 0.75 Angstrom(-1), the param eters obtained from the refinement of the neutron data and from struct ure factors obtained from a multipole deformation refinement. Conventi onal spherical atom refinement on 4030 reflections (F-o greater than o r equal to 3 sigma F-o) gives R=0.037 and R(w)=0.038, while the multip ole refinement on 3627 reflections gives R=0.028 and R(w)=0.024. A pro nounced aspherical electron distribution has been found for the cobalt atom in the deformation maps which is consistent with a significant t rigonal distortion of the distribution expected for an octahedral liga nd field, viz. t(2g)(5.0) and e(g)(2.0). The populations obtained from the multipole refinement confirm this and give apparent orbital occup ancies of 3.7(1), 1.2(1) and 2.4(1) for the e(g)(t(2g)(+/-)), a(g)(t(2 g)(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 orbi tals. The results are consistent with a (4)A(g) ground state for the C o2+ ion, which is in accord with the various earlier magnetic, spectro scopic and heat capacity studies.