The Organometallic Reader

Dedicated to the teaching and learning of modern organometallic chemistry.

Periodic Trends of the Transition Metals

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Periodic trends play a huge role in organic chemistry. Regular changes in electronegativity, atomic size, ionization energy, and other variables across the periodic table allow us to make systematic predictions about the behavior of similar compounds. Of course, the same is true for organometallic complexes! With a firm grip on the periodic trends of the transition metals, we can begin to make comparisons between complexes we’re familiar with and those we’ve never seen before. Periodic trends essentially provide an exponential increase in predictive power. In this post, we’ll hit on the major periodic trends of the transition metals and discuss a few examples for which these trends can be handy.

Before beginning, a couple of caveats are in order. First of all, many of the trends across the transition series are not perfectly regular. Hartwig wisely advises that one should consider the transition series in blocks instead of as a whole when considering periodic trends. For instance, general increases in a quantity may be punctuated by sudden decreases; in such a case, we may say that the quantity increases generally, but definite conclusions are only possible when the metals under comparison are close to one another in the periodic table (and we need to be careful about unexpected jumps). Secondly, periodic trends are significantly affected by the identity of ligands and the oxidation state of the metal center, so comparisons need to be appropriately controlled. Using periodic trends to compare a Pd(II) complex and a Ru(III) complex is largely an exercise in futility, but comparing Pt(II) and Pd(II) complexes with similar ligand sets is reasonable. Keep these ideas in mind to avoid spinning your wheels unnecessarily! Alright, let’s dive in… Read the rest of this entry »

Written by Michael Evans

January 9, 2012 at 12:56 am

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

Simplifying the Organometallic Complex (Part 1)

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Organometallic complexes, which consist of centrally located metals and peripheral organic compounds called ligands, are the workhorses of organometallic chemistry. Just like organic intermediates, understanding something about the structure of these molecules tells us a great deal about their expected reactivity. Some we would expect to be stable, and others definitely not! A big part of our early explorations will involve describing, systematically, the principles that govern the stability of organometallic complexes. From the outset, I will say that these principles are not set in stone and are best applied to well controlled comparisons. Nonetheless the principles are definitely worth talking about, because they form the foundation of everything else we’ll discuss. Let’s begin by exploring the general characteristics of organometallic complexes and identifying three key classes of organic ligands.

When we think of metals we usually think of electropositive atoms or even positively charged ions, and many of the metals of OM chemistry fit this mold. In general, it is useful to imagine organic ligands as electron donors and metals as electron acceptors. When looking at a pair of electrons shared between a transition metal and main-group atom (or hydrogen), I imagine the cationic metal center and anionic main-group atom racing toward one another from oblivion like star-crossed lovers. In the opposite direction (with an important caveat that we’ll address soon), we can imagine ripping apart metal–R covalent bonds and giving both electrons of the bond to the organic atom. This heterolytic bond cleavage method reproduces the starting charges on the metal and ligand. Unsurprisingly, the metal is positive and the ligand negative.

Behold, it is the east! And carbanion is the sun!

Behold, it is the east! And carbanion is the sun!

Read the rest of this entry »

Written by Michael Evans

January 3, 2012 at 12:57 am

Resources for Organometallic Chemistry

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What resources are available for the interested organometallics student?

What resources are available for the interested organometallics student?

Let’s face it: organometallic chemistry is a somewhat esoteric subject. Unfortunately, this fact makes it difficult to find cheap, current textbooks on the subject, but there are a few used gems for sale on the Internet. Crabtree’s Organometallic Chemistry of the Transition Metals is a short but solid book that’s a good jumping-off point for deeper studies. Spessard and Miessler’s Organometallic Chemistry is a longer but still informative classic. Hartwig’s “biblical” Organotransition Metal Chemistry is a nice reference work, but I wouldn’t start off with this back-breaking tome. If you do, skip around and avoid the vast sections of text describing “what’s known” with little explanation.

For the penny-pinching student or layman, there are several good resources for organometallic chemistry on the Web. Nothing as exhaustive as Reusch’s Virtual Textbook of Organic Chemistry exists for organometallic chemistry, but the base of resources available on the Web is growing. Rob Toreki’s Organometallic HyperTextBook could use a CSS refresh, but contains some nice introductions to different organometallic concepts and reactions. Try the electron-counting quiz!

VIPER is a collection of electronic resources for teaching and learning inorganic chemistry, and includes a nice section on organometallic chemistry featuring laboratory assignments, lecture notes, and classroom activities. Awesome public lecture notes are available from Budzelaar at the University of Manitoba and Shaughnessy at Alabama (Roll Tide?). For practice problems, check out Fu’s OpenCourseWare material from MIT and Shaughnessy’s problem sets.

What did I miss? Feel free to contribute your favorite resources in the comments below. Keep an eye on the right-hand sidebar for a growing collection of online OM chemistry resources.

Written by Michael Evans

January 1, 2012 at 12:33 pm

What is Organometallic Chemistry?

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Let’s begin with a few simple questions: what is organometallic chemistry? What, after studying organometallic chemistry, will we know about the world that we didn’t know before? Why is the subject worth studying? And what kinds of problems is the subject meant to address? The purpose of this post is to give the best answers I currently know of to these questions. The goal of this otherwise content-free post is twofold: (1) to help motivate us as we move forward (that is, to constantly remind us that there is a point to all this!); and (2) to illustrate the kinds of problems we’ll be able to address using concepts from the field. You might be surprised by the spine-chilling power you feel after learning about the behavior of organometallic compounds and reactions!

Put most bluntly, organometallic (OM) chemistry is the study of compounds containing, and reactions involving, metal-carbon bonds. The metal-carbon bond may be transient or temporary, but if one exists during a reaction or in a compound of interest, we’re squarely in the domain of organometallic chemistry. Despite the denotational importance of the M-C bond, bonds between metals and the other common elements of organic chemistry also appear in OM chemistry: metal-nitrogen, metal-oxygen, metal-halogen, and even metal-hydrogen bonds all play a role. Metals cover a vast swath of the periodic table and include the alkali metals (group 1), alkali earth metals (group 2), transition metals (groups 3-12), the main group metals (groups 13-15, “under the stairs”), and the lanthanides and actinides. We will focus most prominently on the behavior of the transition metals, so called because they cover the transition between the electropositive group 2 elements and the more electron-rich main group elements.

Why is the subject worth studying? Well, for me, it mostly comes down to synthetic flexibility. There’s a reason the “organo” comes first in “organometallic chemistry”—our goal is usually the creation of new bonds in organic compounds. The metals tend to just be along for the ride (although their influence, obviously, is essential). And the fact is that you can do things with organometallic chemistry that you cannot do using straight-up organic chemistry. Case in point:

The venerable Suzuki reaction...unthinkable without palladium!

The venerable Suzuki reaction...unthinkable without palladium!

The establishment of the bond between the phenyl rings through a means other than dumb luck seems unthinkable to the organic chemist, but it’s natural for the palladium-equipped metal-organicker. Bromobenzene looks like a potential electrophile at the bromine-bearing carbon, and if you’re familiar with hydroboration you might see phenylboronic acid as a potential nucleophile at the boron-bearing carbon. Catalytic palladium makes it all happen! Organometallic chemistry is full of these mind-bending transformations, and can expand the synthetic toolbox of the organic chemist considerably.

To throw another motive into the mix for the non-specialist (or the synthesis-spurning chemist), organometallic chemistry is full of intriguing stories of scientific inquiry and discovery. Exploring how researchers take a new organometallic reaction from “ooh pretty” to strong predictive power is instructive for anyone interested in “how science works,” in a practical sense. We’ll examine a number of classical experiments in organometallic chemistry, both for their value to the field and their contributions to the general nature of scientific inquiry.

What kinds of problems should we be able to address as we move forward? Here’s a bulleted list of the most commonly encountered types of problems in an organometallic chemistry course:

    • Describe the structure of an organometallic complex…
    • Predict the product of the given reaction conditions…
    • Draw a reasonable mechanism based on evidence…
    • Devise a synthetic route to synthesize a target organometallic compound…
    • Explain the observation(s)…
    • Predict the results of a series of experiments…

    The first four are pretty standard organic-esque problems, but it’s the last two, more general classes that really make organometallic chemistry compelling. Just imagine putting yourself in the shoes of the pioneers and making the same predictions they did!

    There you have it, a short introduction to organometallic chemistry and why it’s worth studying. Of course, we’ll use the remainder of space in the blog to fully describe what organometallic chemistry really is…but it’s helpful to keep these motives in mind as you study. Keep a thirst for predictive power, and it’s hard to go wrong with organometallic chemistry!

    Written by Michael Evans

    December 31, 2011 at 9:44 am

    Welcome!

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    Welcome! The Organometallic Reader is devoted to the teaching and learning of organometallic chemistry. Quite simply, I believe that there is a better way to teach organometallic chemistry than the approach taken by the average American graduate school. Plus, you don’t need to be a chemistry whiz kid to learn organometallic chemistry! John Anthony said it best:

    You don’t need to be a genius to do chemistry; you just need to be smarter than molecules.

    In many respects, the molecules of organometallic chemistry are unlike any you encounter in general, organic, or inorganic chemistry. Still, they are definitely governed by general principles that come into greater focus with each passing day, and you can learn a lot about the other branches of chemistry by studying organometallics. Few fields cross so many of the traditional “divisions” of chemistry.

    In the coming weeks, we’ll explore the general nature of organometallic chemistry as a whole and its signature molecules, organometallic complexes. We’ll get acquainted with periodic trends of the transition metals, that large but often forgotten chunk of elements stuck in the center of the periodic table. From there, we’ll round out organometallic structure with discussions of the different classes of ligands commonly found in OM complexes. We’ll then move into the elementary mechanistic steps of organometallic chemistry, and finally we’ll put them together as we discuss organometallic reactions. Get ready for a wild ride!

    Written by Michael Evans

    December 30, 2011 at 5:36 pm