Metal complexes have a variety of structures. Silver complexes are often linear; beryllium complexes are usually tetrahedral; iron forms a carbonyl compound that has a trigonal bipyramidal structure; cobalt(III) complexes are octahedral; and tantalum forms an eight-coordinated fluoride complex (Figure 3.1). Although a variety of coordination numbers and structures have been observed in metal complexes, the only common coordination numbers are four and six; the common structures corresponding to these coordination numbers are tetrahedral and square planar, and octahedral, respectively. In studying metal complexes, it soon becomes clear that the octahedral structure is by far the most common of these configurations.
An interesting and useful approach to the prediction of structure of molecules is given by the valence shell electron-pair repulsion (VSEPR). It, however, applies to molecules in which there are no valence d electrons, and is, 45
Figure 3.1 Illustrations of structures found in metal complexes.
therefore, not applicable to transition metal complexes. CFT provides the simplest explanation of the effect of d electrons on the structure of complexes.
CFT notes that d orbitals have a specific geometry and orientation in space and claims that d electrons will residue in the orbitals that are farthest from neighboring atoms or molecules. The presence of d electrons in six- or four-coordinate complexes may cause distortion of the expected octahedral or tetrahedral configuration. The distortion arises because ligands will avoid those areas around a metal ion in which the d electrons reside.
Let us consider the influence of metal d electrons on structure. If there are zero, five (unpaired), or ten d electrons present in the outer d subshell of an atom, there is no distortion of the structure of its complexes. This is true because empty, half-filled, and filled d subshells have spherical electrical symmetry; a charged particle (for example, a ligand) on a sphere having the metal at its center will encounter the same electrostatic force regardless of its position on the sphere. Therefore, the position that a ligand will occupy is not influenced by d electrons in these cases.
The complex [Ti(H2O)6]3+ contains one d electron; this electron will repel ligands that are near it. If this complex has the expected octahedral geometry, the one electron should be in a t2g orbital, one which points between H2O ligands. If the electron were in the dxyorbital, one would expect the ligands in the xy plane to be repelled. This would lead to four long Ti—OH2 bonds (and two short ones along the z axis). Four long bonds and two short ones would also be expected if the electron were placed in a dxzor dyzorbital. (The reader should convince himself of this.)
Since the t2gorbitals point between ligands, one might expect that the effect of the presence of electrons in these orbitals would be small. In fact, unequal bond lengths in complexes such as [Ti(H2O)6]3+ have not been detected by X-ray diffraction studies; however, the visible spectra of these complexes do provide evidence of a slight distortion. In octahedral d3 complexes, such as [Cr(H2O)6]3+, each t2gorbital contains one electron. Each of the six ligands in an octahedral array would be near two of these d electrons, and hence all would experience the same repulsion. No distortion is expected or observed.
In [Cr(H2O)6]2+, which is a d4high-spin system, the first three electrons go in t2g orbitals and produce no distortion of an octahedral structure. The fourth electron goes in an eg orbital that points directly at ligands. If the electron resides in a dz2 orbital, the ligands on the z axis are repelled; if it resides in the dx2–y2 orbital, the four ligands in the xy plane are repelled. In fact, six-coordinated d4metal complexes have distorted structures in all cases studied.
For example, in MnF3 each manganese(III) is surrounded by six F– ions so arranged that four are closer to the Mn3+ ion than are the other two (Figure 3.2).
Figure 3.2 An example of a Jahn–Teller distortion. An octahedral molecule distorts to a tetragonal shape.
We have now considered the distortion of octahedral structures caused by the presence of 0, 1, 2, 3, 4, 5 (unpaired), and 10d electrons. It should be clear that high-spin d6, d7, d8, and d9systems are similar to d1, d2, d3, and d4systems, respectively. Six-coordinated complexes of d9 metal ions exhibit distortions similar to those of d4 complexes. The most common examples are copper(II) complexes. In [Cu(NH3)4]2+, the tetragonal distortion is so marked that the square planar tetraammine complex results. Note, however, that solvent molecules occupy positions above and below the plane in solutions of complexes of this type; these solvent molecules are farther from the metal ion than are groups in the square plane. The distortion of symmetrical structures resulting from partially filled electronic energy levels (in this case the d sublevel) are called Jahn–Teller distortions.
The distortions of octahedral structure observed in important low-spin configurations should also be considered. Low-spin d6systems are similar to d3 complexes. The six electrons completely fill the t2gorbitals. Since each of the six ligands is close to two of these orbitals there is no tendency for distortion
d1, d6 Tetragonal distortion Not observed d2, d7 Tetragonal distortion Not observed
d3, d8 No distortion Experimentally verified
d4, d9 Large tetragonal distortion Experimentally verified
d5, d10 No distortion Experimentally verified
Low-spin:
d6 No distortion Experimentally verified
d8 Large tetragonal distortion Square planar compounds
We have considered the distortions to octahedral structure that result from the presence of d electrons. Tetrahedral structures are also observed in metal complexes; however, they are less common than octahedral and distorted octahedral configurations. If four ligands surround a metal atom, a tetrahedral structure is expected. Two exceptions must be noted. As we have seen, four-coordinated low-spin d8 complexes are square planar, as are many four-coordinated d9and high-spin d4complexes. Tetrahedral d3, d4, d8, and d9systems should exhibit marked Jahn–Teller distortions; however, very few examples of this type of compound exist. Low-spin tetrahedral complexes need not be discussed, since there are no examples of such complexes. The tetrahedral crystal field splitting (ǻt) is apparently too small to cause spin pairing.
Although it is possible to predict fairly accurately the stereochemistry of complex ions in which the coordination number of the central atom is known, it is much more difficult to predict the coordination number of the central atom.
Large coordination numbers are favored by the electrostatic attraction of negatively charged ligands (or polar molecules) for a positive metal ion.
Covalent bonding theories predict, in general, that the greater the number of bonds formed to an element, the greater is the stability of the resulting compound.
The tendency for large coordination numbers is opposed by steric and electrostatic (or Pauli) repulsion between ligands. No simple scheme has been
presented which makes predictions from these criteria. Note, however, that the first-row transition elements are frequently six-coordinated. Four-coordination is observed primarily in complexes containing several large anions, such as Cl–, Br–, I– and O2–, or bulky neutral molecules.
The most common and important complex ions are hydrated metal ions.
The coordination numbers and structures of some of these simple complexes have been determined. Isotope dilution techniques were used to show that Cr3+ and Al3+ are bonded rather firmly to six water molecules in aqueous solutions. The interpretation of the visible spectra of solutions of transition metal ions using CFT indicates that ions such as V2+, Mn2+, Fe2+, Co2+, Ni2+, V3+, Cr3+, and Fe3+ are octahedral [M(H2O)6]m+ species. For non-transition metal ions it has been more difficult to obtain structural information. However, nuclear magnetic resonance spectroscopy demonstrates that Be2+ in aqueous solution is surrounded by four water molecules. These data support the importance of six coordination. The only exception cited here is Be2+, an element which obeys the octet rule.