Four- and Six-Coordinate Preferences

FIGURE 10.30 Anguiar Overiap Energies of Four- and Six-Coordinate Compiexes across a Transition Series. 

Oniy sigma bonding is considered. (a) Octahedrai and square-pianar geometries, both strong- and weak-fleld cases. 

The energies of all of these bonding molecular orbitals depend on the energies of the metal atomic orbitals (approximated by their orbital potential energies) and the ligand orbitals. 

FIGURE 10.31 Simulated Hydration Enthalpies of Transition-Metal Ions. 

An alternate way to examine these preferences is to look for trends within the vast collection of known four-coordinate metal complexes. Alvarez and coworkers analyzed the structures of more than 13,000 four-coordinate transition-metal complexes and reported these trends (1) (f, d, (f, (f, and d configurations prefer the tetrahedral geometry, (2) d and (f complexes show a strong preference for the square planar geometry, (3) d, ct, cfi, and metals appear in either tetrahedral or square planar structures, (4) a significant fraction of ions have structures intermediate between square planar and tetrahedral, and (5) a large number of structures that cannot be adequately described as tetrahedral, square planar, or intermediate are found for d, cfi, and d complexes. These trends build on the angular overlap-derived preferences. 

In many cases, Cu(II) complexes have square-planar or nearly square-planar geometry, 

Angular overlap calculations of the energies expeeted for different numbers of d electrons and different geometries can give us some indication of relative stabilities. Here, we will eonsider the three major geometries, octahedral, square planar, and tetrahedral. In Chapter 12, similar calculations will be used to help describe reaetions at the coordination sites. 

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