The Organometallic Reader

Dedicated to the teaching and learning of modern organometallic chemistry.

Posts Tagged ‘number of d electrons

Dissociative Ligand Substitution

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Associative substitution is unlikely for saturated, 18-electron complexes—coordination of another ligand would produce a 20-electron intermediate. For 18-electron complexes, dissociative substitution mechanisms involving 16-electron intermediates are more likely. In a slow step with positive entropy of activation, the departing ligand leaves, generating a coordinatively unsaturated intermediate. The incoming ligand then enters the coordination sphere of the metal to generate the product. For the remainder of this post, we’ll focus on the kinetics of the reaction and the nature of the unsaturated intermediate (which influences the stereochemistry of the reaction). The reverse of the first step, re-coordination of the departing ligand (rate constant k–1), is often competitive with dissociation.

A general scheme for dissociative ligand substitution.

A general scheme for dissociative ligand substitution. There’s more to the intermediate than meets the eye!

Reaction Kinetics

Let’s begin with the general situation in which k1 and k–1 are similar in magnitude. Since k1 is rate limiting, k2 is assumed to be much larger than k1 and k–1. Most importantly, we need to assume that variation in the concentration of the unsaturated intermediate is essentially zero. This is called the steady state approximation, and it allows us to set up an equation that relates reaction rate to observable concentrations Hold onto that for a second; first, we can use step 2 to establish a preliminary rate expression.

(1)     rate = k2[LnM–◊][Li]

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Epic Ligand Survey: Carbenes

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Epic Ligand Survey: CarbenesIn a previous post, we were introduced to the N-heterocyclic carbenes, a special class of carbene best envisioned as an L-type ligand. In this post, we’ll investigate other classes of carbenes, which are all characterized by a metal-carbon double bond. Fischer carbenes, Shrock carbenes, and vinylidenes are usually actor ligands, but they may be either nucleophilic or electrophilic, depending on the nature of the R groups and metal. In addition, these ligands present some interesting synthetic problems: because free carbenes are quite unstable, ligand substitution doesn’t cut the mustard for metal carbene synthesis. Off we go!

General Properties

Metal carbenes all possess a metal-carbon double bond. That’s kind of a given. What’s interesting for us about this double bond is that there are multiple ways to deconstruct it to determine the metal’s oxidation state and number of d electrons. We could give one pair of electrons to the metal center and one to the ligand, as we did for the NHCs. This procedure nicely illustrates why compounds containing M=C bonds are called “metal carbenoids”—the deconstructed ligand is an L-type carbenoid. Alternatively, we could give both pairs of electrons to the ligand and think of it as an X2-type ligand. The appropriate procedure depends on the ligand’s substituents and the electronic nature of the metal. The figure below summarizes the two deconstruction procedures.

The proper method of deconstruction depends on the electronic nature of the ligand and metal.

The proper method of deconstruction depends on the electronic nature of the ligand and metal.

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Ligand Field Theory & Frontier Molecular Orbital Theory

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In this post, we’ll begin to explore the molecular orbital theory of organometallic complexes. Some background in molecular orbital theory will be beneficial; an understanding of organic frontier molecular orbital theory is particularly helpful. Check out Fukui’s Nobel Prize lecture for an introduction to FMO theory. The theories described here try to address how the approach of ligands to a transition metal center modifies the electronics of the metal and ligands. The last post on geometry touched on these ideas a little, but we’ll dig a little deeper here. Notably, we need to address the often forgotten influence of the metal on the ligands—how might a metal modify the reactivity of organic ligands?

Ligand Field Theory

The ligand field theory (LFT) fleshes out the ideas of crystal field theory with molecular orbital theory concepts. It provides a method for understanding M–L bonding and antibonding orbitals; however, it has been strongly disputed by computational studies in favor of valence bond models that incorporate hypervalency. Still, LFT provides a more complete picture of complex bonding than crystal field theory, so we’ll discuss it here. Furthermore, the portions of LFT under dispute have nothing to do with CFT, so “no harm no foul.” Let’s take a look at the molecular orbitals of a hypothetical octahedral MLn complex to begin hashing out LFT.

Notice that the M–L bonding orbitals mostly have ligand character, while the antibonding orbitals mostly reside on the metal.

The M–L bonding molecular orbitals mostly have ligand character, while the antibonding orbitals mostly reside on the metal.

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Gee, I’m a Tree: Predicting the Geometry of Organometallic Complexes

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An important issue that we’ve glossed over until now concerns what organometallic complexes actually look like: what are their typical geometries? Can we use any of the “bookkeeping metrics” we’ve explored so far to reliably predict geometry? The answer to the latter questions is a refreshing but qualified “yes.” In this post, we’ll explore the possibilities for complex geometry and develop some general guidelines for predicting geometry. In the process we’ll enlist the aid of a powerful theoretical ally, crystal field theory (CFT). CFT provides some intuitive explanations for geometry the geometry of OM complexes. Here we go!

Because OM complexes feature a variety of coordination numbers, the possibilities for geometry are numerous. The common geometries of organic chemistry—linear, pyramidal, trigonal planar, and tetrahedral—are available to OM complexes too. Many complexes exhibit a second kind of four-coordinate geometry, square planar. Five-coordinate complexes may exhibit either square pyramidal or (my personal favorite) trigonal bipyramidal geometries. Six-coordinate complexes feature either octahedral geometry or the rare but intriguing trigonal prismatic arrangement. The figure below summarizes these possibilities (minus the two-coordinate geometries, which we won’t deal with).

Common geometries of organometallic complexes.

Common geometries of organometallic complexes.

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Written by Michael Evans

January 10, 2012 at 12:54 pm

Simplifying the Organometallic Complex (Part 3)

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So far, we’ve seen how deconstruction can reveal useful “bookkeeping” properties of organometallic complexes: number of electrons donated by ligands, coordination number, oxidation state, and d electron count (to name a few). Now, let’s bring everything together and discuss total electron count, the sum of non-bonding and bonding electrons associated with the metal center. Like oxidation state, total electron count can reveal the likely reactivity of OM complexes—in fact, it is often more powerful than oxidation state for making predictions. We’ll see that there is a definite norm for total electron count, and when a complex deviates from that norm, reactions are likely to happen.

Let’s begin with yet another new complex. This molecule features the common and important cyclopentadienyl and carbon monoxide ligands, along with an X-type ethyl ligand.

What's the total electron count of this d6, Fe(II) complex?

What is the total electron count of this Fe(II) complex?

The Cp or cyclopentadienyl ligand is a polydentate, six-electron L2X ligand. The two pi bonds of the free anion are dative, L-type ligands, which we’ll see again in a future post on ligands bound through pi bonds. Think of the electrons of the pi bond as the source of a dative bond to the metal. Since both electrons come from the ligand, the pi bonds are L-type binders. The anionic carbon in Cp is a fairly standard, anionic X-type binder. The carbon monoxide ligands are interesting examples of two-electron L-type ligands—notice that the free ligands are neutral, so these are considered L-type! Carbon monoxide is an intriguing ligand that can teach us a great deal about metal-ligand bonding in OM complexes…but more on that later. Read the rest of this entry »

Written by Michael Evans

January 6, 2012 at 1:40 pm

Simplifying the Organometallic Complex (Part 2)

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Now it’s time to turn our attention to the metal center, and focus on what the deconstruction process can tell us about the nature of the metal in organometallic complexes. We’ll hold off on a description of periodic trends of the transition series, but now is a good time to introduce the general characteristics of the transition metals. Check out groups 3-12 in the table below.

The transition metals are colored dark blue in this table.

The transition metals are colored dark blue in this table.

The transition metals occupy the d-block of the periodic table, meaning that, as we move from left to right across the transition series, electrons are added to the d atomic orbitals. Just like organic elements, the transition metals form bonds using only their valence electrons. But when working with the transition metals, we need to concern ourselves only with the d atomic orbitals, as none of the other valence subshells contain any electrons. Although the periodic table may lead you to believe that the transition metals possess filled s subshells, we imagine metals in organometallic complexes as possessing valence electrons in d orbitals only! The reason for this is somewhat complicated, but has to do with the partial positive charge of complexed metals. Neutral transition metal atoms do, in fact, possess filled s subshells. Why, then, is it important to remember that the valence electrons of complexed metal centers are all d electrons? We will see that the number of d electrons possessed by a complexed metal is in many ways a useful concept. If you find that your counts are off by two, this common mistake is probably the culprit! Read the rest of this entry »

Written by Michael Evans

January 4, 2012 at 3:41 pm