Electron emission spectra obtained by thermal collisions of He*(2 S-3) metastable atoms with CO on Ni(100) in the c(2x2) structure and on Ni(111) in the c(4x2) structure were measured to probe directly the spatial electron distribution. For a systematic comparison, the metastable spectra of free CO, condensed CO on Ni(111), and gaseous Cr(CO)(6) were also measured under the same beam conditions. Our data showed that the relative ionization cross sections for the CO 4 sigma-, 1 pi-, and 5 sigma -derived states depend drastically on the molecular orientation of CO with respect to the metastable beam, reflecting the local electron density of CO in the impact region. Moreover, it was found that the 4 sigma- and 5 sigma-derived states of CO at hollow sites on Ni(111) are strongly modified in space by mixing with each other, where considerable charge transfer occurs from the C site to the O site in the 5 sigma -derived state and in the opposite way in the 4 sigma -derived state. In contrast, such a strong charge redistribution was not seen in the cases of terminal CO on Ni(100) and Cr(CO)(6). These findings were in good accordance with the crystal orbital overlap population obtained by density functional theory through a generalized gradient approximation.