As 4-electron ligand

From all the data summarized in this review it clearly appears that the potential of phospholes in coordination chemistry is enormous. As 2-electron ligands their behavior is already original. Contrary to what could be expected on the basis of their potentially aromatic structure, metal-phosphole bonds are strong due to three contributing factors ... 

A. Monoacetylene, mononuclear complexes acetylenes as 2-electron ligands... 

Other examples of complexes with acetylenes as 2-electron ligands are jr-CjHsCoPhaPPhC Ph [13a], IrCl(C0)(Ph3P)2PhC C02Et[136]and r-C5H5VPhfeCPh(CO)2( ) [13c]. 

Almost all stable carbenes behave as 2-electron-CT-donating ligands with a few exceptions. In particular, in almost all cases corresponding Rh(I) complexes were targeted due to the easy synthetic method. An exception is the cyclopropenylidene carbene, with an extremely acute carbene angle. In this case, a second equivalent of carbene squeezes into the rhodium center, eliminating a chloride anion, giving the cationic dicarbenic rhodium(l) complex [51] (Scheme 5). 

We have excluded the widely studied dibenzophospholes from this review because they have no free dienic system and, thus, act only as classical 2-electron ligands. But, we have included a section on arsole and stibole complexes, and a brief comparison between azametallocenes and their phosphorus analogs in order to illustrate the influence of the heteroatom upon the properties of these systems. This review is intended to be a complete coverage of the literature up to October 1981. 

IIL5. Phospholyls as (5 + 2)-Electron Ligands - Complexes of Phosphametallocenes... 

As with their phosphorus analogs, azametallocenes can play the role of 2-electron ligands by using the lone pair on nitrogen ... 

The synthesis of multimetallic transition metal complexes where the metals are held at specific distances from each other is an important objective because of their potential role in multimetal-centered catalysis in both biological and industrial reactions (1). Moreover, such systems, through cooperative electronic and/or steric effects between metal centers, might give rise to distinct reactivity patterns for both their stoichiometric and catalytic reactions, which are not available to their monometallic analogues (2). Of the ligands that are able to maintain the metal centers in close proximity, the pyrazolate ion (pz ) appears to be a particularly suitable candidate. Pyrazoles (Hpz ) are weak bases (3, 4) and behave as 2-monohapto ligands. 

Alkenes, R2C=CR2, tend to bond to metal centres in a side-on (i.e. T] ) manner and behave as 2-electron donors. The metal-ligand bonding can be described in terms of the Dewar-Chatt-Duncanson model (Figure 23.5). The C=C TT-bonding MO acts as an electron donor, while the 7t -MO is an electron acceptor. Populating the 7t -MO leads to ... 

Addition of O2 to give an q -peroxo complex is related to reaction type 23.30. Each addition in equations 23.29-23.31 occurs at a 16-electron metal centre, taking it to an 18-electron centre in the product. Most commonly, the precursor has a or r/ conhguration, e.g. Rh(I), Ir(I), Pd(0), Pd(II), Pt(0), Pt(II), and the metal must have an accessible higher oxidation state, e.g. Rh(III). If the starting compound contains an 18-electron metal centre, oxidative addition cannot occur without loss of a 2-electron ligand as in reaction 23.32. 

Serving as a strongly bound ancdlaiy ligand wifli certain electronic properties and effects to be imposed on the residual coordination sphere, functioning then mainly as 2-electron NO ligand or as a 3-electron donor in the neutral form... 

Some simpler ligands can also serve as ir-donors. We saw earlier that acetylenes can act as CT-donors and ir-acceptors in the same way as olefins. However, the rr-bonding orbitals that are perpendicular to those oriented toward the metal can also serve as ir-donors, as shown in Figure 1.29. Acetylene ligands in complexes that possess less than 18 electrons without such TT-donation are sometimes considered to be "four-electron donors." This is the origin of the listing of alkynes as 2-electron or 4-electrori ligands in Table 1.1. 

Table 3.11. Theoretical Ability of Clusters to Form Bonds Expressed as Numbers of 2-Electron Ligands per One Rhodium Atom...

It might be expected that azulenes would be able to act as 7-electron ligands. In those complexes whose crystal structures have been determined this is not so. In the binuclear iron complex, (azuIenejFe CCOjj, for example, azulene acts as a 5 + 3 electron ligand while in the similar molybdenum compound, (azulenejMo CCO), it is a 2 x 5 electron ligand. 

A monohapto doubly bridging carbonyl still functions as a 2 electrons ligand the lone pair orbital on carbon can overlap with a suitable in phase combination of filled metal orbitals, while an out of phase combination of filled metal orbitals can interact with the CO n orbital (Figure 23). The net result is to reduce the CO bond order to about 2. 

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