Ligand acceptor/donor properties

FACTORS AFFECTING LIGAND REACTIVITY 1.29.2.1 Electron-acceptor/donor Properties of the Metal Center... 

The EAN rule can also be explained in terms of molecular orbital theory. Housing of 18 electrons requires nine molecular orbitals, the HOMO of which should preferably be bonding in character. To produce these molecular orbitals, the metal combines all its valence orbitals with the ligand orbitals. The symmetry of the complex and the acceptor-donor properties of the ligands determine whether or not the HOMO is bonding, nonbonding, or antibonding in character. 

Abstract The theoretical and experimental research on carbodiphosphoranes C(PR3)2 and related compounds CL2, both as free molecules and as ligands in transition metal complexes, is reviewed. Carbodiphosphoranes are examples of divalent carbon(O) compounds CL2 which have peculiar donor properties that are due to the fact that the central carbon atom has two lone electron pairs. The bonding situation is best described in terms of L C L donor acceptor interactions which distinguishes CL2 compounds (carbones) from divalent carbon(ll) compounds (carbenes) through the number of lone electron pairs. The stmctures and stabilities of transition metal complexes with ligands CL2 can be understood and predictions can be made considering the double donor ability of the carbone compounds. 

The theoretical work showed that the rotational barriers of the vinylidene ligand increased with X from n acceptor, a donor to n donor properties. Ligands (X) with... 

Overall, the review deals mainly with the chemistry in aqueous media, with occasional mention to work in organic solvents. Cyanometallate complexes are known to display profound changes in their electronic structure and reactivity when dissolved in solvents with different acceptor capability, associated with the donor properties of the exposed electron pairs at the cyano ligands (15). These specific interactions are also related to the role of cationic association in the thermodynamics and kinetics of the reactions involving cyano complexes (16). 

The redox potentials of many metalloporphyrin complexes are sensitive to the nature of the electrolyte in which they are measured. This is because one or both axial positions are coordinated by either solvent or anionic ligands. Such dependencies of E° values have been extensively studied by Kadish and coworkers 29 notably good correlation between E° data and Donor/Acceptor Number properties of solvents have been observed. 

The unique electronic structure of these (L-A3)MoO(dithiolene) complexes arises from two basic factors. The first is the strong axial a- and Ji-donor properties of the terminal oxo ligand, which dominates the ligand field and predetermines the energy of the Mo-based dxz, dyz, and dzi acceptor orbitals. The second is the equatorial dithiolene sulfur donors, from which the low-energy LMCT transitions arise. Dithiolene covalency contributions to the electroactive C, or redox, orbital can be directly probed via the relative oscillator strengths of the / —> ixy and /fp —> (/", transitions (see above). These three wave functions may be expanded in terms of Mo- and dithiolene sulfur-based functions ... 

The vast majority of the coordination compounds of Os that have been prepared are in the oxidation states 11 and III. Moreover, many of these compounds show reversible or well defined Os / couples in which the electronic and redox properties at the metal are controlled by the a-donor, 7r-acceptor, and r-donor properties of the ligands. Indeed, the study of the redox behavior in Os / and Ru / species, metal ions in which octahedral coordination is almost universally retained in both redox partners, has been central in recent developments to parameterize metal centered redox processes as a function of ligand donor and acceptor capacity. The chemistries of Os and Os are, therefore, intimately linked, and have been extended to studies of important mixed valence Os / binuclear and polynuclear species (see Mixed Valence Compounds). For the purposes of brevity and convenience, this section will deal with Os and Os complexes together. The extensive literature on Os / complexes has been developed with a very wide range of donor ligands a comprehensive assessment of this work is beyond the scope of this article, and the reader is directed to published comprehensive reviews. " ... 

The electron acceptor and donor properties of a metal center relative to a particular ligand depend, apart from the ligand itself, on a variety of factors associated with the metal (such as its position in the periodic table, its oxidation state and coordination number), to the co-ligands (e.g., their electronic donor/acceptor ability) and to the overall coordination entity (namely the electron count and the net charge). Naturally, all such factors also affect the reactivity of the ligand which is therefore determined by a complexity of combined effects whose relative weight is often not easy to predict. 

A range of five- and six-coordinate rhodium(I) and rhodium(III) mono-, bisand tris(boryl) systems have been reported by Marder and co-workers, which provide a useful platform to discuss the fundamental properties of boryl ligands, notably their trans influence (a donor properties) and potential as 7T-acceptors. The structural features of several of these systems were reviewed in 1998 [11] and these (along with new systems) are assessed in the light of more recent theoretical studies [15,175]. 

Substituent groups on the olefin with a +1 effect (e.g. Me) will tend to strengthen the M- -ol o-bond and weaken the 7r-bond while -I substituents should have the opposite effect. The tendency for the thermodynamic stability of Ag+—ol complexes to increase as the +1 nature of substituents increases supports the claim that o-bonding is more important than ir-back-bonding. However, there is also some evidence that the jr-acceptor properties of a ligand are more important than its o-donor properties 75). 

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