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Typical Ligands Total Electron Count

Main group elements like C and S have 4 valence AOs, one s and three p, and they follow the octet rule (although heavier main group elements can extend their octet). Transition metals, by contrast, have 10 valence AOs—one s, five d, and three p, in that order—and they follow the 18-electron rule. The 18-electron rule is much less rigorous for transition metals than the octet rule is for main-group elements. First, it can be difficult to surround a metal, especially an early metal, with sufficient numbers of substituents to provide 18 electrons to the metal. Second, the valence orbitals of metals are sufficiently extended from the nucleus that the nucleus doesn t care much about what s going on in its valence shell. [Pg.257]

It s more difficult to obtain a count of the total number of electrons around a metal than it is around a main-group element. Unlike in main-group compounds, where substituents almost always bring one or two electrons to an atom, substituents (or ligands) attached to metals can provide anywhere from one to six electrons to the metal. However, just a few classes of ligands are widely used in organometallic chemistry, and it s not too hard to remember how many electrons each one donates. [Pg.257]

Alkenes and alkynes can also act as Lewis bases toward metals by using the two electrons in their 77 bonds. Such an interaction is called a tt complex. The metal-alkene bond is a c bond due to its spherical symmetry around the axis formed by the metal and the midpoint of the C=C tt bond. Even m bonds such as the H—H and C—H bonds can act as two-electron donors to metals in this mcuiner, and these compounds are called a complexes. When a cr complex is formed in an intramolecular fashion, the bond is called an agostic bond or agos- [Pg.257]

The preceding pair of resonance descriptions is called the Dewar-Chatt-Duncanson model of rr bonding. Similar pictures can be drawn for metal-alkyne complexes, regardless of whether the second tt bond is acting as a tt donor, and for metal-1,3-diene complexes. [Pg.259]

A similar picture can be drawn for a complexes. H2 can use the two electrons in its cr bond to form a a complex with a metal, M—(H2). If the metal has at least two valence electrons, it can back-bond to the r orbital, lengthening and weakening the H—bond until the complex is better described as a dihydride complex H—M—H. In fact, a continuum of H2 complexes of metals has been discovered from one extreme to the other. Polyhydride complexes continue to be a very active area of research. [Pg.259]


See other pages where Typical Ligands Total Electron Count is mentioned: [Pg.272]    [Pg.257]    [Pg.272]    [Pg.257]    [Pg.189]    [Pg.29]    [Pg.381]    [Pg.1089]   


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Counts total

Electron counts

Electron total

Electronic counting

Total electron count

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