thumbnail ~ k e t c h e ~ Some Unusual Properties of Carbon Monoxide: A Comparison with N2 Thomas J. Venanzi College ot New Rochelle New Rochelle. NY 10801
C a r h m monoxide, CO, is a molecule which has unusual chemical and physical properties. Unlike its inert isoelectronic analog, N2, CO is a reactive and highly poisonous gas. On the other hand, the normal h i l i n g point of this apparently polar molecule is -19Z°C, a value very close to t h e hoiling point of the nonpolar molecule, Nil (-19fi°C). Hoth properties of CO can be partly explained in terms of the usual electronic picture of CO, namely An important feature of the ahove structure is the presence of the lone pair of electrons on the carhon atom. In fact, with the exception of the isocyanides, CO is the rarest of all molecules-a molecule containing a carhon atom with a lone pair. Due t o the lower nuclear charge . of the carhon atom, the attraction hetween the lone pair and the carbon nucleus in CO is not as stronr as the analorous attraction between the lone pairs and the nitrogen nuclei in N2. In this simple picture, the less tightly held lone pair on the carhon atom is primarily responsihle for the reactivity of CO toward electron-seeking metal species such as the nickel atom in the formation of NilCOi". This ~ ~ - reactivitv exists desoite the uresence of an extremely stable triple hond in CO (255 Kcallmol), a bond whirh is even more st,ahle than Lhe unreactive triple hond in N2 (225 Kcallmol). T h e other structural ~eculiaritvof the CO molecule is its polarity. Experimental determination of the dipole moment of CO yields a value of the order of 0.1 Dehye, one of the smallest values for a molecule which is structurally polar. Thus, CO is virtually a nonpolar molecule with the only intermolecular attraction due to dispersion forces. Hence, its boiling point should he very similar to its isoelectronic analog, Nz. One reason for the low polarity of CO lies in the electronegativity oC the carhon atom. Although on the normal electronegativity scale, oxygen has a higher electronegativity than carbon, this is predicted on the fact that carhan forms single honds which are approximately 25% s in character. However, it is generally recognized that as the amount of s character in a bond increases, the electrons in that hond are held more tightly by the atoms involved. Thus, the electronegativity of ~
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edited by MARYVIRGINIA ORNA, O.S.U. College of New Rocheile New Rochelle, NY 10801
that atom is enhanced. In CO, the oxygen atom contributes mostly p orbitals to the t,riple bond, while the carhon atom nominally amtributes two purep orbitals and a hybrid orbital which is 50%s in character. Thus, in contrast to the oxygen atom, the carhon atom has increased its electronegativity. As a result, the electronegativity of the carbon atom in CO has a higher value than its usual value of 2.5. This fact not only means that carbon has an electronegativity value close to that of oxygen. but also it explains to a degree the low polarity of the CO molct:ule. From molecular arhital calculations, another reason far the low polarity of CO can he found in the electron distrihutions in the n and n bonds. In CO, the n bonding plprtron distribution is shifted toward the oxveen (as exresulting low dipole moment of the CO molecule. All in all, the case of carhon mmoxide illustrates the danger in making any predictions from a consideration of the simple electronegativity scale without first investigating the exact nature of the orbital structure of the honds. In recent years, more sophisticated electronegativity scales have been proposed. Any interested reader is urged to write to the authrr for further information on electronegativity models and their possible applicat,ion to CO.
Brief descriptions of phenomena. topics, facts, etc., which chemical educators have found to be of interest in their teaching. will be presented in a "note tvoe" format throuohout the JouRNaL.
She is an adtive member of many professionai organizations inciuding ACS. ' . DIVCHED. HIST, Sigma Xi, the Inter-Society Color Council, and the International institute far the Conservation of Historic and Adistic
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been responsible tor organizing symposiums on color technoiogy and
enjoys constructing crossword puzzles and has been a contributor to Scio~esrand the daily and Sunday New York Times.
Volume 58
Number 5
May 1981
423
