Transition metal complexes geometries

The irreducible representations for the p- and d-orbitals for a central metal atom in some common transition metal complex geometries were discussed in Section 5.8. We are now... 

Click Coached Problems for a self-study module on the geometry of transition metal complexes. 

In all these discussions, we separate, as best we might, the effects of the d electrons upon the bonding electrons from the effects of the bonding electrons upon the d electrons. The latter takes us into crystal- and ligand-field theories, the former into the steric roles of d electrons and the geometries of transition-metal complexes. Both sides of the coin are relevant in the energetics of transition-metal chemistry, as is described in later chapters. 

Stable Mn(HI) compounds, Mn(R2r fc)3, have been known for a long time (42, 46). The structure of Mn(Et2C tc)3 is elucidated (47). The inner geometry of the Mn(CS2)3 core does not conform to the usual D3 point symmetry of transition metal complexes of this type, but shows a strong distortion attributed to the Jahn-Teller effect. The electronic spectrum (48, 49) and the magnetic properties of this type of complexes are well studied (50). 

The metal complexes discussed thus far bear little resemblance to the vast majority of common transition-metal complexes. Transition-metal chemistry is dominated by octahedral, square-planar, and tetrahedral coordination geometries, mixed ligand sets, and adherence to the 18-electron rule. The following three sections introduce donor-acceptor interactions that, although not unique to bonding in the d block, make the chemistry of the transition metals so distinctive. 

The redox ability of a metal complex will be considered in the context of its molecular orbital composition and spin state. In this regard, Figure 1 shows the molecular orbital diagrams for the most common geometries encountered in transition metal complexes. 

In summary, the geometry of transition metal complexes is determined by the necessity (1) to group the ligands about the metal to minimize electrostatic repulsions and (2) to allow overlap of the metal and ligand orbitals. The first... 

Transition metal complexes of sterically hindered thiolate ligands have been reviewed. " These ligands give rise to unusual geometries and oxidation states, and low coordination numbers in their complexes. 

Other unsaturated boron heterocycles, such as borazines and borabenzenes, form transition metal complexes with the expected nido geometry, as exemplified by compounds (Et3N3B3Et3)Cr(CO)3 (123) and (CBH5BPh)Mn(CO)s (110) (Fig. 29). 

The VCD of amino acids and transition metal complexes of amino acids has been the subject of ongoing investigation in our laboratory (82-90), from which has emeiged new information on solution geometries and on the ring current mechanism for generating intense monosignate VCD intensity. 

Structurally, it would be of interest to examine the geometry of the transition-metal complexes 113, formed from 111 and 112. Like 109 and 110, complex 113 should conceivably also be planar. Nevertheless, from a realistic viewpoint, the syntheses of 111 and 112, together with their subsequent metal complexation, seemed to be too ambitious a project. Fortunately, in 1986, Xiu Chun Wang, from the Shenyang College of Pharmacy in the northeastern part of Mainland China, was assigned to synthesize and to complete the follow-up transformations of 111 and 112 to 113. 

---