Intro to Ligand Bonding

Hi, everyone! It’s a lovely Saturday morning (ish), and I don’t know about  you guys, but I’m about ready to get back to school and just finish the semester already. Sure, I have a lot of physics homework to do, and yeah, I should probably go back and review for my Japanese test, too, but that can wait for a little bit longer.

Why, you ask? Because today, we’re going to talk about something that I’ve been itching to learn about since high school chemistry!

Yup! We’re finally going to talk about transition metals!

Main group elements are wholly too predictable. As you know, they all have a fixed valence (carbon is tetravalent carbon is tetravalent cARBON IS TETRAVALENT). If we know their valence configuration, we know how many bonds they’ll make. Even with the expanded octets. Done. End of story.

However, transition metals think that’s a little boring. Instead of adhering strictly to one or two valences, they kind of just do what they want, accepting dizzying numbers of electrons without as much as blinking. What is this madness? Is it witchcraft? Is it a lie of freshman chemistry?

No, it’s coordination chemistry!

Coordination chemistry describes the chemistry of coordination complexes, also called “transition metal complexes.” This is essentially Lewis acid-base chemistry on steroids, and it’s a lot more interesting than the “give one, take one” bonding chemistry that’s the bread and butter of organic chemists.

In transition metal chemistry, the metal acts as a Lewis acid, accepting electrons from basic substituents called ligands. We’ll talk about how to predict the “valence” of transition metals a little later, but first, let’s talk a little bit more about the basics of this ligand-metal bonding.

Ligands are defined based on the number of “teeth” they have, or how many bonds they can form to a metal center. This is called the denticity of a ligand. If a ligand can donate one electron pair, it’s monodentate. Two? Bidentate. Three? Tridentate. You get the idea.

Now, the denticity of a ligand isn’t necessarily equivalent to the number of atoms in a ligand that participate in bonding. (We’ll see this when we get to cyclopentadienyl—it donates six electrons, but five of its atoms participate in the donation.) In order to designate how many atoms in the ligand participate in its bonding to a metal center, we place a η# before its ligand name, where # = the number of participating atoms. (For example, cyclopentadienyl is η5-cyclopentadienyl.)

(Haha, I used the same example as my professor. I didn’t even mean to…)

Polydentate ligands can further be divided into two classes: chelating and bridging. Chelating ligands are ligands that form multiple bonds to the same metal center. Bridging ligands form multiple bonds, but to different metal centers—these are designated with the Greek letter μ before their name.

Now, as far as bonding in general goes, as I said above, it usually happens by donation of a pair of electrons from a ligand to a metal. However, there is another kind of bonding that takes place in these complexes: π backbonding.

π backbonding occurs when empty orbitals (p, d, or antibonding π orbitals) on the donating ligand accept electron density from the metal. This can change a lot of things about the complex. (For example, pi backbonding into antibonding orbitals can weaken metal-ligand bonds.)

π bonding doesn’t just go one way, either. Ligands can also donate electron density from their π orbitals to metals. You thinking what I’m thinking? Yup, alkenes and alkynes can be ligands, too!

Wow, this is a really short post… (600 words, which is like, half my normal length). Still, as far as introductions go, that’s really all you need to get started. Next, we’ll put together a little ligand toolkit, and then we’ll use it to investigate why transition metals act so strangely. Somewhere amid all that, we’ll also learn about protein catabolism (once I post the other two biochem posts I’ve already written TTuTT). Aren’t you psyched??

Questions? Comments? Put ’em below! Stress? Well, here’s a perfectly normal video of some cute kittehs…


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