Interligand repulsions

Octacoordination is often encountered in lanthanide complexes. The preferred poly-hedra for eight coordination expected on the basis of interligand repulsivities are square antiprism (D ), dodecahedron with triangular faces (Z)2d), bicapped octahedron (D3(i), truncated octahedron (Z)2ft), 4,4-bicapped trigonal prism (C2v), distorted cube (C2v), and cube (0/,). The most commonly observed polyhedra for this coordination number are, however, the square antiprism and the dodecahedron. 

Bis(2 -methoxyphenyl)-l,10-phenanthroline, bmpphen (2), forms a high-spin bis- but not a low-spin tris-ligand iron(II) complex due to interligand repulsion. ... 

All of the complexes are six-coordinate high-spin. A trans octahedral structure has been found, for example, in the two complexes [Ni(N02)2(NH3)4]1641 and [Ni(N02)2(en)2] (213).1642 This type of coordination is usually found in complexes where the ancillary ligands do not exert much interligand repulsion. On the other hand, when some steric repulsion is present in the amine ligands O-bonded nitrito coordination usually occurs, as exemplified by the complex [Ni(ONO)2(W,iV-Me2en)2] (214).1642... 

In the case of eight-coordination, the cube is fairly rare the compounds Na3[MFg] (M = Pa, U, Np) constitute the best-established examples. As for the trigonal prism versus the octahedron, interligand repulsion can be reduced by distortion of the cube to yield the square antiprism or dodecahedron (this is best demonstrated by playing with models). These two are about equally common. For example, in the case of Mo(CN)g-both square antiprismatic and dodecahedral coordination are found in the solid state in solution, the ion is stereochemically nonrigid (or fluxional) and appears to convert rapidly from one geometry to the other. 

Steric bulk. The ligands are packed around the metal ion in such a way as to minimize interligand repulsion, and the coordination number is determined... 

Lanthanide complexes with octa-coordination are well-known. The relative stabilities of various coordination polyhedra based on interligand repulsivities are given in Table 5.8. 

Table 19.4 lists coordination environments associated with coordination numbers between 2 and 9 not all are predictable using the Kepert model. For example, after considering the repulsions between the cyano ligands in [Cu(CN)3] , the coordination sphere would be predicted to be trigonal planar (19.2). Indeed, this is what is found experimentally. The other option in Table 19.4 is trigonal pyramidal, but this does not minimize interligand repulsions. One of the most important classes of structure for which the Kepert model does not predict the correct answer is that of the square planar complex, and here electronic effects are usually the controlling factor, as we discuss in Section... 

---