Geometry of Transition Metal Complexes

Metal-metal bonds in polynuclear complexes containing more than four metals are not appropriately described as localized bonds. Wade and Mingos have developed a scheme (supported by theory ) for predicting polyhedral shapes by counting skeletal electron pairs and Teo has described a topological electron-coimting scheme. - These schemes are beyond the scope of this book. 

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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. 

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... 

VAL-BOND An empirical method for calculating and predicting geometries of transition metal complexes. 

N. Rosch, and R. Hoffmann, Geometry of Transition Metal Complexes with Ethylene or Allyl Groups as the Only Ligands, Inorg. Chem. 13, 2656-2666 (1974). 

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 multifaceted behavior of transition metal complexes calls for not only theoretical explanations within a common conceptual framework but also theoretical tools that are powerful enough to predict the chemical and magnetic behavior of open-shell transition metal ions. Specifically, one looks for theoretical methods to calculate geometries and relative energies for stable species and transition states as well as for methods that allow one to determine spectroscopic parameters with sufficiently predictive accuracy. 

An important part of the inorganic chemist s contribution has been the preparation and characterization of transition metal complexes with unsaturated boron rings as ligands. They have also settled the geometry of a number of compounds using X-ray methods. 

Proton NMR spectra of some V1" and Cr1" complexes indicated a facial octahedral configuration with all three sulfur atoms cis.234 More recent 13C and 19F NMR data on a wide range of transition metal complexes of fluorinated monothio-/ -diketones support the assignment of cis square-planar and facial octahedral geometries.235,236 X-Ray structural data have established the cis square-planar configuration for a Pd" and a Pt" complex237 and four Ni" complexes,238,239 the tetrahedral configura-... 

The earliest attempts at determining the structures of transition metal complexes involved an indirect approach, in which H atom positions are inferred from the geometry of the remainder of the molecule. This technique is still heavily used today. 

In the realm of all-carbon ligands in the formation of transition-metal complexes, the naked carbon atom holds a special position. Based on the geometry of metal-carbon interaction, these compounds can be divided into four classes terminal carbide (I), 1,3-dimetallaallene (II), C-metalated carbyne (III), and carbido cluster (IV) ... 

An extensive series of investigations of -transition metal complexes of dithiolenes commenced in the 1960s, stimulated by their facile redox chemistry, the intriguing noninnocence of these ligands, and the novel, trigonal-prismatic geometry of tris(dithiolene) complexes (59). 

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