In the calculation of the electronic structure of transition metal carbonyls it is most expedient to include all of the valence atomic orbitals of the metal, carbon, and oxygen atoms in the basis set. In many cases, however, and, in particular, in the study of the cluster carbonyls of the d-dements, such a basis turns out to be too large. Hence the necessity arises,in the solution of this problem, of using in the basis set those few molecular orbitals of the CO group which are the most involved in the formation of the metal--ligand (M-CO) bonds in place of all the carbon and oxygen valence AO's. When using this procedure it would appear to us that it would be correct to use the molecular orbitals corresponding to the actual electronic structure of the carbonyl groups in the metal carbonyl understudy rather than molecular orbitals characterizing the electronic structure of the free CO group (dc_o = 1,128 A). In order to elucidate the problem formulated above, we have calculated the electronic structure of the Re(CO) s radical using two methods. 1) The basis set consisted of the 6s, 6p, and 5d AO's of Re, the 2s and 2p oxygen AO's,and 2s and 2p carbon AO's. 2) The basis set consisted of the 6s, 6p, and 5d AO's of Re and the MO's of the CO groups which are the most substantially involved in the formation of the M-CO bonds. The calculations were carried out using the extended Htickel method (EHM) [1]. In order to use the MO's of the CO groups as basis functions in calculating the electronic structure of transition metal carbonyls, it is necessary to know the magnitude of the energies of the corresponding MO's and the values of the coefficients for the 2s and 2p oxygen and carbon functions which enter into these MO's. For this purpose we have carried out preliminary caIculations on the electronic structure of the isolated CO group for various different values of d C_O* since in passing from the free CO molecule to the carbonyl, the main effect * The interatomic distances chosen were those observed in the binuclear carbonyls Mz(CO)10 (M = Mn, Tc, Re) [2,3].