ELECTRONIC BASIS OF BOND FORMATION 23 



a number of orbitals of e(|ual energy are available, electrons tend to 

 occupy them singly, keeping their spins parallel. This is indicated 

 in the case of all the metallic ions of Table II, pairing of electrons 

 taking place in the 3r/ shell only when each orbital is already occupied 

 by one electron. 



Both ionic and covalent bonding of the metal atoms occur. In the 

 former case, the electronic structure is as shown in the left-hand 

 section of the table, this type of bond leaving the M orbitals unaf- 

 fected. In consequence ionic bonds are associated with relatively 

 large numbers of unpaired electrons. The metal is held in combina- 

 tion by the electrostatic attraction between its own ionic charge 

 and the charge on the surrounding nitrogen atoms of the complex. 

 The source of the latter charge will be discussed in specific cases in 

 Chapter V, Section 1. 



An atom can form an electron pair (covalent) bond for each avail- 

 able stable orbital, these being in general those of the valency shell. 

 However, in the case of the elements listed in Table II, the orbitals 

 of the Sd level have much the same energy as those of the 4* and 4p 

 (valency) shell. In the metallic ions these 3d orbitals are occupied 

 in many cases only by single electrons, by the pairing of which one 

 or more 3c? orbitals can become available for covalent bond formation. 



When, as in the present cases, there are a number of electron 

 orbitals available of but slightly differing stability, it is found that 

 "hybridization" takes place, resulting in the production of sym- 

 metrically disposed, energetically equivalent bond orbitals, giving 

 bonds of greater strength than any of the component bond orbitals. 

 There are only certain combinations of electronic orbitals for which 

 hybridization produces increased bond strength; thus, in the case of 

 all the ions of Table II except cobaltous and nickelous, hj'bridization 

 occurs among two 3d, one 45 and three 4p electronic orbitals, giving 

 six bond orbitals directed toward the corners of a regular octahedron. 

 Hybrid bond orbitals are designated by the letters representing the 

 quantum levels involved, with the number of electronic orbitals in 

 each case attached as a superscript. Thus, in the example just given, 

 the bonds are referred to as of d^sp^ type. In the case of cobaltous 

 and nickelous ions, only one 3d orbital is available for covalent bond 

 formation. It is found that the strongest bonds which can be formed 

 in this case arise from hybridization of this orbital with the 45 and 

 two 4p orbitals, giving four bond orbitals of type dsp-, directed toward 

 the corners of a square. One 4p orbital is unused. Similarly, although 



