COVALENCIES OF CR IN CR(CO)(6) AND (CO)(4 )ICRCCH3

The electronic structures and bonding of Cr(CO)(6) and (CO)(4)ICrCCH3 were studied with the methods of DV-X(alpha) SCC and natural bond orbi tals (NBOs), and then the covalencies of Cr in these two compounds wer e discussed on the basis of the new difinition of covalence proposed b y one of the authors. The results show that there exists a Cr-C-carbyn e triple bond in which sigma bond is localized and two pi bonds are pa rtially delocalized to carbonyls; on the. other hand, there is no back donation of charges from the central chromium atom to the carbyne car bon atom, so the carbyne carbon possesses lower electronic density tha n carbonyl carbon and methyl carbon in the same molecule, which is con sistent with C-13 NMR spectrum. The covalencies of Cr in both compound s are 12.