Metal—ligand bonds Subject

Although the subject of stability of complexes will be discussed in greater detail in Chapter 19 it is appropriate to note here some of the general characteristics of the metal-ligand bond. One of the most relevant principles in this consideration is the hard-soft interaction principle. Metal-ligand bonds are acid-base interactions in the Lewis sense, so the principles discussed in Sections 9.6 and 9.8 apply to these interactions. Soft electron donors in which the donor atom is sulfur or phosphorus form more stable complexes with soft metal ions such as Pt2+ or Ag+, or with metal atoms. Hard electron donors such as H20, NH3( or F generally form stable complexes with hard metal ions like Cr3+ or Co3+. 

Although essentially within the spirit of ligand field theory as enunciated in introductory remarks, there is an approach to dealing with the metal-ligand bonding which has developed into a field of its own, and deserves separate treatment. It is the so-called Angular Overlap Model (AOM). The choice of name arose from the early ways iif which its procedures were applied, and is no longer particularly apt. Nevertheless the name persists and is likely to do so, and will be employed here. Some of the reasons for the initial choice of the name will become obvious as the subject is outlined. 

The acquisition and analysis of metal-ligand bond energy information in organometallic molecules represents an active and important research area in modern chemistry. This overview begins with a brief historical introduction to the subject, followed by a discussion of basic principles, experimental methodology, and issues, and concludes with an overview of the Symposium Series volume organization and contents. 

In this contribution we shall present several applications of the new method, which we shall refer to as LSD/NL, to the calculation of bond energies of transition metal complexes. We shall focus on trends along a transition period and/or down a transition triad. The following subjects will be discussed a) metal-metal bonds in dimers of the group 6 transition metals b) metal-ligand bonds in early and late transition metal complexes c) the relative strength of metal-hydrogen and metal-methyl bond in transition metal complexes d) the metal-carbonyl bond in hexa- penta-and tetra-carbonyl complexes. 

The way in which metal-ligand bond energies of early transition metals and f-block elements differ from those of middle to late transition metals, or metal-ligand bond energies of 3d and 4f elements differ from those of their heavier congeners, has been the subject of many experimental (75) as well as a few theoretical studies (16) over the past decade. 

Ligands bonded to a metal can undergo a number of structural changes that do not involve complete breaking of the metal-ligand bond(s). Such processes are the subject of the following sections. 

TM compounds in high and low oxidation states with metal-carbon double and triple bonds have been the subject of a systematic theoretical study. Table 7 shows the calculated metal-ligand bond lengths of several tungsten carbene and carbyne complexes at the HF and MP2 levels of theory. The HF optimizations were carried out using two different basis sets. Basis set I has DZ quality at tungsten (no additional 6p... 

There is a large and growing field of transition metal chemistry in which silicon-containing ligands are involved. The object of this review is to provide a guide to the literature on those aspects of the subject described by the title and to deal in detail with topics not treated specifically elsewhere. Section II is concerned with complexes having Si-transition metal (M) bonds, Section III with the role of transition metal complexes in hydrosilylation, and Section IV with complexes having Si—C—M bonds. 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

For the supported catalyst it is expected that the ligand does not leach since it is chemically bonded to the carrier. In contrast, the rhodium metal bound to the ligand is subject to leaching due to the reversible nature of the complex formation. The amount will depend on the equilibrium between rhodium dissolved in the organic phase and that bound to the ligand. When an equilibrium concentration of 10 ppb Rh is attained, the yearly loss of Rh for a 100 kton production plant will be about 1 kg Rh per year. Compared to the reactor contents of rhodium (see Table 3.9, 70 kg Rh) this would result in a loss of 1.5% of the inventory per year, which would be acceptable. 

Ligand substitution reactions of NO leading to metal-nitrosyl bond formation were first quantitatively studied for metalloporphyrins, (M(Por)), and heme proteins a few decades ago (20), and have been the subject of a recent review (20d). Despite the large volume of work, systematic mechanistic studies have been limited. As with the Rum(salen) complexes discussed above, photoexcitation of met allop or phyr in nitrosyls results in labilization of NO. In such studies, laser flash photolysis is used to labilize NO from a M(Por)(NO) precursor, and subsequent relaxation of the non-steady state system back to equilibrium (Eq. (9)) is monitored spectroscopically. 

Since the first report of a complex involving a direct metal-to-Ceo bond, (Ph3P)2 Pt( 7 -C6o)/ numerous studies have established that the fullerene Ceo acts as a moderately electronegative alkene in coordinating to electron-rich metal centers. In many cases the Ceo ligand is subject to relatively facile displacement when the complex is in solution however, the zerovalent, octahedral complexes M(CO)3(dppe)(Cgo) [M = Mo, W dppe = l,2-bis(diphenylphosphino)ethane] display outstanding stability even under severe conditions. The overall time needed to prepare these complexes from commercially available M(CO)g is dramatically reduced by adopting a biphasic procedure for the synthesis of the precursor M(CO)4(dppe), which was first described for the preparation of Mo(CO)4 (dppe). Here details are presented for the biphasic synthesis of W(CO)4(dppe) and for its use in the preparation of W(CO)3(dppe)(Cgo)-... 

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