Recent Advances in the Chemistry of Inorganic Nitrogen Compounds
INTRODUCTION TO SYMPOSIUM
THE
chemistry of the compounds of nitrogen has throughout the history of modern rhemical science provided an area of widespread interest to chemists a t all levels and in almost all fields, both theoretical and applied. The element provides a tremendous variety in the physical aud chemical characteristics of its compounds and in the broad field of their application. The importance of nitrogen compounds in the production of exvlosives. both industrial and militarv. " . and in the manuracture of fertilizers has served to keep this element near thc renter of industrial chemical development, both in peace time and during war. Even when we restrict our attention to relatively simple inorganic molecules we find a great variety in which nitrogen is pre.sent in various oxidation states. These compounds include a number of strong oxidizing agents, e.g., nit.rir acid and nitrogen dioxide, some strong reducing agents, e.g., hydrazine, some of the more interesting nonaqueous solvents, e.g., ammonia and dinitrogen tetroxide, as well as compounds covering a wide range of st,ability and a wide range of biochemical toxicity. As an introduction to this symposium, certain general characteristies of the nitrogen family and the element nitrogen will he summarized. GENERAL CHARACTERISTICS OF THE NITROGEN FAMILY
E'lectronic Conjig~irationand Chemical Bonding. The outer shell of elect,rons in a free atom of each of the elements of the nitrogen family in the normal state has the configuration nsznpa. This configuration leads readily to the possibility of a covalence of three with orhit,als of the pa t.ype, or of the sp3 type with an spa orbital occupied by an unshared electron pair. A covalence of four follows readily from this configuration by the use of all four spa hybrid orbitals for bonding. The heavier elements show covalences of five using spad trigonal bipyramidal orbitals, and covalences of six using sp3dZ octahedral orbitals. Ionic structures of the [ns2np6Ia-type are known, especially for nitrogen and phosphorus. Characteristic oxidation states include -3, +3, and +5, as exemplified by the hydrogen compounds, the oxides, and the halides: MH3, MXI, M2Oa(MlOe),MXb, and M,OS(M,OIO). Because of the relatively high ionization potentials of the elements of the nitrogen family, there is relatively little tendency for the lighter members of this family to form positive ions. The heavier members of the family form M+3 ions, in which presumably the three n p electrons are lost. The family exhibit,^ the characteristic gradation from VOLUME 34, NO. 11, NOVEMBER, 1957
nonmetallic to metallic properties with increasing atomic number as exemplified by increasing basicity (or decreasing acidity) of the oxides in the sequence from nitrogen to bismuth, and by decreasing stability of the binary hydrogen compounds. The physical properties of the free elements change in the same sequence. SPECIAL CHARACTERISTICS OF NITROGEN
Like the first members of the other main groups of the periodic system, nitrogen differs considerably from other members of its family. This difference is related to the following parameters: High electronegativity, considerably higher than other members of the family (N= 3.0 on the Pauling scale, P = 2.1). Ability to form a bonds using p orbitals, i.e., to give double bonds of the familiar type. Lack of availability of d orhitals for a bonding. Thus, multiple bonding is impossible for a nitrogen atom which has formed four o bonds. The extraordinarily high stability of the N, molecule. This contributes to a generally lower thermodynamic stability of nitrogen compounds than otherwise would be the case. The nitrogen molecule has been represented by the electronic formula : N e N:
which involves a r bond plus 2 a bonds. I n terms of molecular orbitals, the nitrogen molecule has the configuration [K,K(~ZS)~(V*ZS)'(,~~)'(*~P, = rr2pJ41 The coordination number of nitrogen and, hence its covalence, islimited to four by the lack of availability of d orbitals in the valence shell of the nitrogen atom. Nitrogen cannot "expand its valence shell." The small size of thenitrogen at,om produces, in many instances, a further decrease in coordination number because of the "radius-ratio effect." Thus, the common form of nitric acid is HxOs, whereas orthophosphoric acid, H3POn, is the familiar form. There is literally not sufficientspace for four oxygen atoms about a single nitrogen atom in the +5 oxidation state. Because of these and other factors, the chemistry of nitrogen and its compounds possesses a complexity and variety which is often bewildering but is always a challenge. The following papers report some of the more recent developments in this fascinating field. ~~
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Chaiman of Symposium
UNIVER~ITV OF FWRIDA ~AINEPVILLE, FLORIDA
