Theories of Bonding in Coordination Compounds

The theories of bonding in coordination compounds [3] have evolved subsequent to Werner s coordination theory (1893). Werner introduced the concept of primary and secondary valency, explaining the formation of the coordination compounds. The 18-electron rule, stating that the stable complexes with low formal oxidation states of metal ions should have 18 bonding electrons around the metal ion, became an important beginning point toward the study of the stabihty of the complexes. The 18-electron rule is significant in modem coordination chemistry as it is also supported by the molecular orbital theory. However, a smaller number of complexes with metals in low oxidation states restrict its wide applicability. An important advance in the theories of bonding in coordination compounds was the introduction of 

Hybridization of atomic orbitals, a mathematical tool for mixing the atomic orbitals to give an equal number of hybrid orbitals with superior directional properties, became a very important feature of this theory in explaining various geometries observed in coordination compounds. 

---

Describe the crystal field theory of bonding in coordination compounds... 

A satisfactory theory of bonding in coordination compounds must account for properties such as color and magnetism, as well as stereochemistry and bond strength. No single theory as yet does all this for us. Rather, several different approaches have been... 

Describe the crystal field theory of bonding in coordination compounds Explain the origin of color in complex species Use the spectrochemical series to explain colors of a series of complexes... 

One night in late 1892, at the age of 26, Alfred Werner was awakened from his sleep at two in the morning. He had just had a dream about the nature of bonding in coordination compounds. For the next 15 h, he recorded his insights in what eventually became his third scientific paper. Unlike the chain theory, Werner postulated that all of the cobalt compounds in Table 15.1 had two types of valences primary (Hauptvalenz) and secondary valence. Furthermore, every metal had a fixed coordination number equal to the number of ligands that were directly bonded to... 

Chapter 11 Coordination Chemistry Bonding, Spectra, and Magnetism 387 Bonding in Coordination Compounds 391 Valence Bond Theory 391 Crystal Field Theory 394 Molecular Orbital Theory 413 Electronic Spectra of Complexes 433 Magnetic Properties of Complexes 459... 

In the following pages, the valence bond theory and the crystal field theory are described very briefly to set more recent developments in their historical context. The rest of the chapter describes the ligand field theory and the method of angular overlap, which can be used to estimate the orbital energy levels. These two supply the basic approach to bonding in coordination compounds for the remainder of the book. 

The insight Werner provided into the bonding in coordination compounds is even more remarkable when we realize that his theory predated Lewis s ideas of covalent bonding by more than 20 years Because of his tremendous contributions to coordination chemistry, Werner was awarded the 1913 Nobel Prize in Chemistry. 

The crystal field, valence-bond, and molecular orbital theories (respectively, CFT, VBT, and MOT) were all viable explanations of the bonding in coordination compounds. CFT treats the M-L interaction as exclusively electrostatic, whereas VBT treats it as the overlap of appropriate native and hybrid atomic orbitals. MOT constructs multinuclear molecular orbitals analogous to the mononuclear atomic orbitals. CFT, particularly with an allowance for some small degree of covalent character, is the most valuable bonding theory available, particularly at the beginning leveL... 

---