Metal-ligand sigma bonds

Some examples of weak-field ligands are CL and Br", which have a valence shell / -orbital, with two electrons in it, oriented perpendicular to the metal-ligand sigma bond. 

The first 12 electrons form six metal-ligand sigma bonds. The J-electrons of the metal enter the Lg- and Cg-orbitals in the same way they did in the crystal field theory. Ligand field theory, however, identifies the t2g-orbitals as nonbonding and the g-orbitals as antibonding. 

Fe forming a metal-to-ligand sigma bond. Tlie sp orbital on carbon contaiiiiiig the lone pair o erlaps with the empty sp orbital on Fe. 

Carbon monoxide binds to Fe in hemoglobin some 200 times more strongly than oxygen. This is the reason why CO is a toxic substance. The metal-to-ligand sigma bond is formed by donating a lone... 

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

In a similar way a sigma bond exerted by a functional surface OH-group to a metal ion causes a trans effect on the ligands bound to the metal ions (cf. Chapter 9.1). 

These reactions resemble those described in section 2.2.1 because both types involve insertion of the transition metal into the Si-H or Ge-H bond. However, here the molecule eliminated is not a neutral ligand but is formed by deinsertion of two ligands which are sigma bonded to the transition metal after the addition of R3MH. 

At this level of approximation, each bond between the metal and a ligand is considered to be a two-electron sigma bond. For the purposes of electron counting (as in the inert-gas rule), this is normally adequate. The number of such bonds is the coordination number. Each single, simple ligand contributes unity to this number. Thus, a square planar coordination implies a coordination number of 4. An octahedron implies a value of 6. The typical values are also included in Table 4.4. 

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