Ir-Acceptors

One additional point should be discussed here, concerning the substantial emphasis that has been placed on the differences between alkyl and aryl isocyanides. It has been suggested, primarily on the basis of infrared evidence, that aryl isocyanides are better 7r-acceptors than alkyl isocyanides (90). Qualitatively this difference is easily rationalized. One can see that delocalization of charge into 7r -orbitals on an aryl ring in aryl isocyanide-metal complexes should be possible, whereas no such possibility exists for alkyl isQcyanide-metal complexes this means that aryl isocyanides should be better ir-acceptors. Of course, the simple qualitative model gives one no measure of the relative importance of this effect. 

Inductive and resonance parameters, (Tr°, ct, were evaluated for the isocyanide ligand from F chemical shifts they suggest that the isocyanide is a good cr-donor and a weak ir-acceptor. 

Transition metal alkyls are often relatively unstable earlier views had attributed this either to an inherently weak M—C bond and/or to the ready homolysis of this bond to produce free radicals. Furthermore, the presence of stabilizing ir-acceptor ligands such as Cp , CO, or RjP was regarded as almost obligatory. However, (1) the M—C bond is not particularly weak compared say to the M—N bond, and (2) the presence of the new type of ligand on the metal could make the complex kinetically stable thus, even isoleptic complexes, i.e., compounds of the form MR , might be accessible 78, 239). These predictions have largely been borne out (see Table VII). 

Preparation and Structure. - Among a number of interesting conclusions drawn from ab initio calculations on the model substituted ylide system (1) is that cr-donor and ir-acceptor... 

A T structure with the strongest ct-donor D trans to the empty site (I in Scheme 1) is preferred in the case of three pure cr-donor ligands. The presence of a ir-acceptor ligand also makes the T structure more stable. When one of the ligands is a tt-donor, X, a Y structure of type II (Scheme 1) is observed. This structure permits the formation of a w bond between the empty metal d orbital and the lone pair of X. No such tt bond is present in the T structure since all symmetry adapted d orbitals are filled. This partial M—X multiple bond stabilizes Y over T. 

Figure 3.84 Effect of a 7r-bonding ligand in acting as a ir-acceptor. (Reproduced with permission from S.A. Cotton and F.A. Hart, The Heavy Transition Elements, published by Macmillan Press...

The ligands are referred to as ir acceptors because of their receiving electron density donated from the metal to 7rg orbitals. Back donation results in increasing the bond order between the metal and ligand, so it results in additional bonding. 

When spectral studies are conducted to see how different ligands function as ir acceptors, it is found that the ability to accept electron density varies in the order... 

Figure 4. Some molecules containing both M-M multiple bonds and ir-acceptor ligands.

In [Ni(py)4X2], pyridine apparently has a larger en than Cl- or Br- (48, 64-67), suggesting that pyridine is a 7r-donor towards Ni(II). However, it will be recalled from 2.1.1. that pyridine is apparently a jr-acceptor in Cr(HI) complexes (58). While it is quite possible that pyridine could have values of opposite sign with different metals, it is surely more likely to function as a ir-acceptor towards Ni(II) than towards Cr(III). We shall comment further on this discrepancy in Section 7. 

A remarkable result is the position of (Proto-DME) between (TPP) and (TpivPP) in Series b, Table 15. The vco values originate from various sources and the observed differences between the three porphyrins may thus be meaningless. (Note the enormous solvent dependence of the infrared spectra of various hemes that has been reported recently (29).) Anyway, the three porphyrins have approximately the same ir-acceptor capacity. Therefore, the tetraarylporphyrin moiety, especially in the picket fence hemes, e.g., Fe(TpivPP)LX [33], is comparable with the natural hemes Fe(Proto-DME)LX ([14], M = Fe), and its use as a model porphyrin for the study of hemoprotein properties is well justified, despite the very different substitution pattern. 

These types of arguments have been used by us for explanation of characteristic peculiarities in spectra of compound 10 NH-tautomers (Fig. 10) and were applied to understand the principal changes in the NH-tautomer visible absorption of investigated porphyrins with non-symmetrical substitution. It may be concluded that difficulties in the theoretical treatment of spectral peculiarities of porphyrins with ir-acceptor substituents ( ) are caused by the absence of detailed absorption and fluorescence excitation spectra of individual tautomers of these compounds. 

Covalent r.-phenyl glycine Brush , ir-acceptor (S)-DNB-phenylglycine... 

Olefins form a great variety of complexes with transition metals. In all these complexes the olefin is bonded to the metal atom by both acceptor bonds and this is favored by a low valence state of the metal atom. 

The potential for cr orbitals to serve as ir acceptors has become apparent in recent times. Phosphines, instead of using empty pure d orbitals as vr acceptors, may accept it donation into low-lying cr orbitals or into hybrids involving tr and 3d... 

Further crystallographic evidence for metal-carbonyl ir bonding is found in phosphine and phosphite derivatives of hexacarfconylchromium- Substitution of R3P for CO in Cr(CO)ft creates a complex of C4 symmetry in which one CO group lies trans to the phosphorus ligand (Fig. 11.29). The two trans ligands will compete for the same ir orbital, but carbon monoxide is a better v acid (ir acceptor) than the phosphine (Fig. 11.30). As a result, the Cr—CO r bond should be shorter relative to Cr—CO and to Cr—CO in CrtCO). The data in Table 11.12 show that these predictions are borne out. in keeping with the substantial ir character in the metal-carbonyl bond. 

It appears that the metal-ligand bonds in carbonyl and dinitrogen complexes are similar but somewhat weaker in the dinitrogen complexes. Carbon monoxide is no only a better cr donor but also a better ir acceptor. This is 10 be expected on the basis of whatever polarity exists in the CO molecule (see Fig. 5.18) and the fact that the ir antibonding orbital is concentrated on the carbon atom (see Fig. 5.20), which favors overlap with the metal orbital. The superior r accepting ability of CO also accounts for the instability of carbonyl dinitrogen complexes. Both Cr(CO)sN2 and cis-... 

In general, the conditions favoring adherence to the 18-electron rule are an electron-rich central metal (e.g.. one that is in a low oxidation state) and ligands that are good ir acceptors.5... 

Complexes of technetium in oxidation states ranging from (-1) to (VII) have been prepared chemically and characterized. However, historically only the higher oxidation states (IV), (V) and (VII) have been of major importance in radiopharmaceutical formulations. More recently there has been increased interest in lower oxidation state technetium complexes for medical applications, and the use of ir-acceptor ligands has allowed the preparation of Tc1 complexes which are stable in vivo. The coordination chemistry of technetium has been described in Chapter 42 and recent reviews have been provided by Davison21 and by Schwochau.22 Reviews which relate to medical applications of technetium are given by Jones and Davison,549 Deutsch et al.,20 Deutsch and Barnett,550 Siedel551 and Clarke and Fackler.552 The in vivo chemistry of "mTc chelates has been described by Eckelman and Volkert,553 while the structures of technetium complexes, determined by X-ray diffraction techniques, have been reviewed by Bandoli et ai554... 

Birch and Pearson22 have studied electrophilic substitution of a triphenylphosphine-substituted system, [Fe(cyclohexadiene)(CO)2PPh3], (8 equation 7). Several features of their results are instructive. First, substitution of CO by the better o-donor, poorer ir-acceptor PPh3 ligand renders the complex more reac-... 

The nitro group in the parent nitrobenzene evidently acts as ir-acceptor, which pulls the electron density out of the aromatic ring. An unpaired electron will obviously occupy the first vacant it-orbital of the nitro fragment (i.e., the lowest-energy-fragment orbital). Interaction between occupied and vacant orbitals is the most favorable. In the nitrobenzene... 

Xem = 371—430 nm). Clearly, changing from a ir-acceptor anionic ligand in (CyNC)AuICN to a ir-donor anionic ligand in (CyNC)AuIX alters the electronic properties of these complexes and, consequently, their luminescence behavior. Note that the luminescence and absorption characteristics of (CyNC) Au X are rather similar to those communicated for the related compound (CO) Auta.67... 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

---