The Electronic Structures and
M. H. Panckhurst University of Otogo Dunedin, New Zeoland
Stereochemistry of NO,', NO,, and NO:
The electronic structures and stereochemistry of the species NOz+, NO2, and NOr- are usually dealt with in textbooks of inorganic chemistry. The experimental information and usually assumed electronic structures are given in Table 1 (only one each of a pair of resonance hybrids is shown for NO2 and NO2-). However, as far as the author is aware, no attempt has been made to relate the electronic structures to the shapes of the species, except for NO2+. Such a relation will be suggested in terms of the stereochemical importance1 of lone pair electrons and of repulsion between electron p a k 2 Table 1
NO*+ - ~. LON0 NO hond length Usually assumed electronic structures
NO?
NO* -
180" 134.1' 115.4" 1 . 1 5 4 i 1 . 1 8 8 i 12361
:0'.
fl
+$:
6:
.?\\
.{I..
-
/'
:N
\\
For NOz+ the valence bond model readily accounts for the linearity of the ion by postulatin~sp hybridization for nitrogen and ?r bonding leading to the strncture in Table 1. The structure shown for NOnwould necessarily2 be non-linear because of repulsion between the nitrogen lone pair and the bonding electrons. For NOz the resonating structure shown, involving a three electron bond as originally suggested by
' SIDGWCK,N. V., AND POWELL,A. M., PTOC.ROY.SOC.,A176, 153 (1940).
1
GILLE~PIE, R. J., AND NYHOLM, R.S., Quart. Reu. (London), W. A., THIS JOURNAL, 34, 187
11, 339 (1957); and FOWLRB, G. (1957).
270
/
Journal o f Chemical Mucation
P a ~ l i n gdoes , ~ not readily account for the non-linearity of the molecule. However this structure can itself be regarded as arising from resonance among structures of the types
in which the unpaired electron is associated with either the oxygen or the nitrogen as in Pauling's discussionJ of the NO molecule. As a result, a "partial lone pair" is located on the nitrogen. One consequence of this model is that electron repulsion would lead to nonlinearity of the molecule and the relation to NOzbecomes apparent. By a similar argument to that employed3 for NO, the bond order in NOZ would be expected to be less than the 2 in NO3+,but greater than the 1'/? in NOp- thus rationalizing the bond lengths for all three species. I n addition the electron distribution about the nitrogen in both NO2 and NOz- can be regarded as essentially triangular-planar with one position occupied by a lone pair or "partial lone pair." The bond angles then arise by distortion of the triangular plane angle of 120'. I n the case of NOa- the well-supported assumptionZ that lone pair-bond pair repulsions are greater than bond pair-bond pair repulsions predicts an angle of less than 120'. For NOz the presence of only a "partial lone pair" on the nitrogen suggests an expansion of the angle. Also, in comparing NO2 and NOz-, the shorter bond length in NOn would lead to greater bond pair-bond pair repulsion than in NOz-. a PAUL IN^, L., "The Nature of the Chemicd Bond," 2nd ed., Oxford University Press, London, 1950, section viii-34.
