As a 7i Acceptor

The chemistry of transition metal complexes bearing a cationic phosphenium ligand dates back to the synthesis and characterization of [(CO)4Fe P(NR2)2 ][PF6] by Parry in 1978.2 Since then, many cationic phosphenium complexes have been investigated because a phosphenium may serve as a a-donor (a Lewis base) and also as a 7i-acceptor (a Lewis acid) due to the lone pair electrons and a vacant p orbital, and a few review articles have appeared.3 7 [LraM(PR2)], being an electrically neutral transition metal complex, is sometimes treated as a phosphenium complex because it can be considered to consist of LraM and PR. 8 11 In this chapter, the focus is on electrically cationic transition metal complexes described as [LraM(PRR )]+ -... 

A phosphenium ligand can be considered to have a vacant p orbital and a positive charge on the phosphorus to some extent even in a transition metal complex (see below). Thus, it serves as a 7i-acceptor resulting in the activation of other 7i-acceptor ligands existing on the same transition metal, and is susceptible to both intermolecular and intramolecular nucleophilic attack causing migration. 

Molecular nitrogen. N, is tsoelectronic with both carbon monoxide and the nitrosyl ion but. despite the numerous complexes of CO and NO, for many years it proved to be impossible to form complexes of dinitrogen. This difference in behavior was usually ascribed to the lack of polarity of N2 and a resultant inability to behave as a 7i acceptor.49... 

CO acts as both a a-donor (via the lone pair of electrons on carbon) and a 7i-acceptor ligand in transition metal complexes. CO is usually depicted as having a triple bond (one a- and two ti-) between the C and the O as well as lone pairs on both the C and the O. The lone pair on C is used for donation into a suitable metal centred o-orbital. However, the strongest M-CO bonds are formed (in simple terms) when some of the electron density donated by the carbon to the metal is directed back from a filled metal (i-orbital of the correct symmetry into an antibonding ti of the CO. Thus the M-CO bond has two parts, the forward (C M) donation, and the (M C) back donation (Figure 1). 

Metal vapour s)mthesis in rare-earth chemistry has been used as early as 1977 to prepare rare-earth butadiene and alk)me complexes by interaction of butadienes and alk)mes (respectively) with vaporised metals (Evans, 1987 Evans et al., 1977), and we have seen in Section 3 that low-valent scandium complexes can also be made by this technique. Additionally, condensation of rare-earth vapours with a 7i-acceptor such as 1,4-di-(t-Butyl)diazadiene (DAD) has produced compounds of general formula [R(DAD)3], except with scandium where the composition is [Sc(DAD)2]. Extensive metal-ligand electron transfer is effective in these molecules so the real oxidation state of the rare earth (at least at room temperature) is in fact + 3. Note that the first structurally characterised organosamarium(II)... 

Thus, the oxidation process could restore in volume the platinum metal and AcQ. In order to verify the validity of the mechanism given in Scheme 2.7, which supposes the concomitant insertion of AcQ via its reduced form, EQCM experiments have been performed. For example, as displayed in Figure 2.32, the addition of a 7i-acceptor to the saturated phase of platinum provides a new mass increase. Moreover, it was established that the mass increase specifically due to the 71-acceptor, is proportional to the initial amount of platinum. 

In the UV spectral range complexation with 18-crown-6 causes a hypsochromic shift of the band with the longest wavelength in various solvents (Bartsch et al., 1976 Hashida and Matsui 1980). Gokel and Cram (1973) reported that complexation with binaphtho-20-crown-6 (11.2) produces a yellow to red color. This phenomenon is very likely to be due to a charge-transfer band between a naphthalene ring as donor (7i-base) and the arenediazonium ion as acceptor (7i-acid). 

The 7i-donor behavior of 4-arylmethylene-2-phenyl-5(47/)-oxazolones 762 with the 7i-acceptor tetracyanoethylene has also been studied. The initially formed charge-transfer complex is converted via intermediate 763 to a new compound for which a 2-aryl-l-benzamido-3,3,4,4-tetracyanocyclobutanecarboxylic acid 764 has been proposed on the basis of the NMR spectral data (Scheme 7.233). Charge-transfer complexes of 2-aryl-4-arylidene-5(47/)-oxazolones with di- and trinitrobenzene as n acceptors have also been prepared. ... 

The experimental data demonstrate that in their transition metal chemistry silylenes 83-85 are able to replace carbonyl, tertiary phosphine or alkene ligands from a metal and it suggests that the silylene behaves as a strong cr-donor and a weak 7i-acceptor. Therefore, it behaves more like the isolobal PR3 ligand than CO. Differences in reactivity between the two silylenes 83 and 85 are due to the different steric requirements. The sterically more flexible ligand is silylene 85. 

---