Theoretical Basis for the Bonding and Properties of Complexes

In this section, we consider two models that address, in different ways, several key features of complexes how metal-ligand bonds form, why certain geometries are preferred, and why these complexes are brightly colored and often paramagnetic. 

Application of Valence Bond Theory to Complex Ions 

Valence bond (VB) theory, which helps explain bonding and structure in main-group compounds (Section 11.1), is also used to describe bonding in complex ions. In the formation of a complex ion, the filled ligand orbital overlaps the empty metal-ion orbital. The ligand (Lewis base) donates the electron pair, and the metal ion (Lewis acid) accepts it to form one of the covalent bonds of the complex ion (Lewis adduct) (Section 18.8). Such a bond, in which one atom in the bond contributes both electrons, is called a coordinate covalent bond, although, once formed, it is identical to any covalent single bond. Recall that the 

The partial orbital diagrams depict the mixing of two 3d, one 4s, and three 4p orbitals h Cr to form six d sp hybrid orbitals, which are filled with six NH3 lone pairs (red). 

VB concept of hybridization proposes the mixing of particular combinations of s, p, and d orbitals to give sets of hybrid orbitals, which have specifie geometries. Similarly, for eoordination eompounds, the model proposes that the number and type of metal-ion hybrid orbitals occupied by ligand lone pairs determine the geometry of the complex ion. Let s diseuss the orbital eombinations that lead to octahedral, square planar, and tetrahedral geometries. 

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Theoretical Basis for the Bonding and Properties of Complexes Vblence Bond Theory Crystal Field Theory Complexes in Biological Systems... 

Viewed as a whole, it appears that the results of the structural, spectroscopic, theoretical, and other physical studies on metal-alkynyl complexes cannot be interpreted within a single, simple picture of the nature of metal-alkynyl bonding. In view of the fact that understanding the electronic-structural basis of the reactions of metal-alkynyl complexes and the physical properties of the advanced materials developed from them requires a clear picture of this bond, further work in this area would be highly desirable. Of particular value would be studies that applied a combination of the experimental techniques described above and theoretical calculations to a series of archetypal metal-alkynyl complexes from across the transition series, so as both to gauge the effect of the metal on the nature of the M-CCR bond and to clarify the benchmark parameters for each technique within different metal-alkynyl bonding regimes. 

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