L2X ligands

If this ligand is bound so that all five atoms are essentially at the same distance fi-om the metal centre (jj -CsH coordination), the five tt electrons are involved in the metal-ligand bonds, so that cyclopentadienyl is classified as an L2X ligand. Two graphical representations are therefore possible, depending on whether one gives a localized or delocalized description of the ligand s tt system (1-6). 

This is generalized for ligands of L X type in the covalent model, which quite naturally become L X " ligands in the ionic model. The cyclopentadienyl radical (Cp), a neutral species with five n electrons (an L2X ligand, 1-5), is therefore considered in its monoanionic form (Cp with six 7T electrons). Table 1.3 presents the numbers of electrons attributed to the principal ligands that have been considered so far in the covalent and ionic models. 

The cyclopentadienyl ligand (Cp) is very widespread in organometal-lic chemistry. When it is coordinated in the ti mode, the most common situation, it behaves as an L2X ligand (Chapter 1, 1.1.1.3). [CpMLj] complexes (L = CO, PR3,. ..) are therefore pseudo-octahedral complexes of the ML5X type, which have 18 electrons when their electronic configuration is 4 . A 4 [CpMLj] complex may therefore be used as a new initial structure, very similar to that used previously. Since the presence of the cyclopentadienyl ligand oxidizes the metal by one unit, a metal with seven valence electrons (M = Mn, Tc, Re) must be used to obtain a neutral complex. The complex [CpMn(CO)3] is therefore a... 

A further simple example of the flexibility of the CBC method that is not afforded by consideration of oxidation numbers is provided by the chloride ligand which serves an X function when coordinated to a 17-electron fragment, e.g. Mn(CO)sCl, but may also act as an LX (or L2X) ligand when coordinated to a < 15-electron fragment and TT-donation is significant, e.g. W(PMc3)4Cl2. 

It is the number of occupied coordination sites on the transition-metal center. Thus, a L or X ligand occupies one site, a L2 ligand (and most often a LX ligand) occupies two sites, and the L3 or L2X ligands occupies 3 sites. When the complex is of the form MLnXp, the coordination number is ... 

The study of rapid, intermolecular ligand exchange between square-planar complexes trans-Ir(CO)L2X (X = C1 or Me, L - PPh3, P(p-tolyl)3, or PMePh2) by variable-temperature 31P NMR spectroscopy indicates that the reaction proceeds through dissociation of phosphine from the metal center and a subsequent associative substitution with other complexes 559,560 Ligand exchange between square-planar Ir and Pt complexes is slow. 

Whereas d8 Ni selects the rectangular di-allylic hyperbonding pattern in (4.121), d6 Fe of ferrocene offers an additional vacant d orbital and hence opens up new geometrical possibilities of an additional cu bond. In concert with the three cu bonds and nominal sd2 (90°) hybridization (Table 4.52), the two Cp ligands are naturally expected to coordinate in r 5 (L2X) fashion to occupy the six octahedrally arrayed coordination sites of the metal. Visualization of this coordination mode is aided by considering the possible patterns of L-type (filled circles 7tcc) and X-type (half-filled circles radical) sites of L2X Cp... 

Applications of tripodal [S3] and [Se3] L2X donor ligands to zinc, cadmium and mercury chemistry Organometallic and bioinorganic perspectives... 

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