Valence Bond Theory

Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable energy. These orbitals accept electron pairs from filled ligand orbitals (Lewis bases) to form coordinate covalent metal-ligand bonds. The type of hybridization and the number of hybrid orbitals determine the geometry of the complex.

In a tetrahedral complex, three vacant p orbitals and one vacant s orbital on the metal hybridize to form four sp³ hybrid orbitals, which overlap with the filled ligand orbitals to form the covalent coordinate bond. Similarly, six hybrid orbitals are created for the octahedral complexes by mixing the vacant atomic orbitals on the central metal ion (d²sp³ or sp³d² hybridization). In the case of linear complexes, the one s and one p orbitals overlap, leading to the formation of two sp hybrid orbitals.

Inner and Outer Orbital Complexes

The strength of the approaching ligands influence the hybridization of the atomic orbitals on the central metal ion. Consider the example of an octahedral complex such as [Co(NH₃)₆]³⁺. The Co³⁺ ion contains six electrons in the 3d orbitals and has vacant 4s and 4p orbitals. The incoming NH₃ ligands, which are strong field ligands, force the unpaired 3d electrons to rearrange and pair up with the other 3d electrons. This creates two vacant 3d orbitals, which combine with one 4s and three 4p orbitals to form six equivalent d²sp³ hybrid orbitals. The six hybrid orbitals overlap with the filled atomic orbitals of the ammonia ligands to form the octahedral complex. Since the inner d (3d) orbitals on the metal participate in hybridization, [Co(NH₃)₆]³⁺ is an inner orbital complex. Due to the absence of unpaired electrons, the complex is diamagnetic, or called a low spin complex.

      In another octahedral complex like [Co(F)₆]³⁺, since the fluoride ligand is a weak field ligand, the 3d⁶ electrons of the metal do not rearrange. To provide vacant orbitals for hybridization, two of the outermost empty 4d orbitals combine with the one 4s and three 4p orbitals to form six vacant hybrid orbitals. Since the outermost d orbitals are used, the hybridization is referred to as sp³d² hybridization, and the complex is called an outer orbital complex. The presence of unpaired electrons makes the complex paramagnetic, and hence these complexes are also known as high spin complexes.

High-spin or outer orbital complexes are more labile and less stable (due to the higher energies of sp³d² orbitals) compared to the low-spin or inner orbital complexes.