Ir-Acceptor ligands

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). 

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

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... 

Ammonia forms a number of complexes with osmium (see Table 5), though the only fully established unsubstituted osmium ammine which has been isolated is the hexaammine [Os(NH3)6]3+ there is, however, electrochemical evidence for the existence both of [Os(NH3)6]2+ and of [Os(NH3)6]4+. Amongst the substituted ammines which have been isolated and characterized are several of osmium(II), all with supporting (or stabilizing) ir-acceptor ligands, e.g. [Os(N-H3)sCO]2 [Os(NH3)s(NO)]3+ and [Os(NH3)5(N2)]2+. 

Compounds containing clusters or M-M multiple bonds often provide synthetic routes to conventional complexes that are superior to others and, in some cases afford products not yet accessible in other ways. This is particularly the case when species with M-M multiple bonds are treated with strong ir-acceptor ligands. Since this subject will be discussed in detail later in this symposium by R. A. Walton, no more will be said here. 

We have already discussed the case of ir-acceptor ligands (CO, NO, etc. in Section 16-3) in which there is at least fractional ir-character based on metal to ligand back donation. In this section we are concerned with bonds in which there are full double and triple bonds. Typical of these are the following ... 

One other binary carbonyl does not obey the rule, the 17-electron VfCO). This complex is one of a few cases in which strong ir-acceptor ligands do not succeed in requiring an 18-electron configuration. In V(CO)6, the vanadium is apparently too small to permit a seventh coordination site hence, no metal-metal bonded dimer, which would give an 18-electron configuration, is possible. However, VfCO) is easily reduced to [VfCO) ], a well-studied 18-electron complex. 

In the presence of phosphines and other ir-acceptor ligands, photolysis of the M2(CO),g produces simple substitution products with intact metal-metal bonds (see Table 2 for examples). In spite of the simple appearance, these substitutions proceed... 

Coupling reactions. Under catalysis of (PhCN)2PdCl2 Negishi coupling performs better with diphenyl(o-chalconyl)phosphine, which is a ir-acceptor ligand. ... 

Another experimental probe of BD versus a donation is Mossbauer spectroscopy of a series of iron complexes. Plots of isomer shift versus quadrupole splitting for [FeH(PPXL)] for L such as Hj, Nj, CO, MeCN, and Cl show that the complexes fell way off the straight line defined by the acceptor ligand than early calculations showed. More quantitative measures of BD are provided by charge decomposition analysis (CDA) and extended transition state (ETS) analysis." ... 

FIGURE 1.7 The effect of turning on the ir interaction between a ir-acceptor ligand and the metal. The unoccupied, and relatively unstable ir orbitals of the ligand are shown on the right. Their effect is to stabilize the filled d, orbitals of the complex and so increase A. In W(CO)6, the lowest three orbitals are filled. 

As a highly reduced metal, Ni(0) prefers ir-acceptor ligands like P(OMe)3. PMe3 as a poor ir acceptor causes the electron density on the metal to rise so much that the NiL3 fragment is a poor a-acceptor. 

In complexes containing good ir-acceptor ligands, an increase in positive charge on the metal does not seem to be accompanied by a substantial increase in A. For example, both Fe(CN)6 - and Fe(CN)6 have A values of approximately 34,000 cm b In the transition from Fe(CN)6 to Fe(CN)6, the ir(ff) level is destabilized just as much as the o ( d) level, probably the result of a decrease in M— Lit bonding when the positive charge on the metal ion is increased. 

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