Spinach contains a lot of iron but is not a good source of dietary iron because nearly all the iron is tied up in the oxalate complex [Fe(C₂O₄)₃]^3-.

(c) Draw a crystal field energy-level diagram for [Fe(C₂O₄)₃]^3-, and predict the number of unpaired electrons. (C₂O₄^2- is a weak-field bidentate ligand.)

Video transcript

Hi, everybody. Welcome back. Let's look at our next problem. Oxalates and food have been linked to a number of health issues including hypercalcemia and kidney stone formation. What is the crystal field energy level diagram four? And we have a complex and overall charge of negative three. It has Cobalt and three oxalate ligands. How many unpaired electrons does it have noted oxalate is a b dentate weak field ligand. So we're going to need to come up with a crystal field energy level diagram and determine how many unpaired electrons we have. So in order to do that, we have to determine how many D electrons we have. So to do that, we need to look at our formula and determine the charge on cobalt and then look at its electron configuration. So we know that our overall charge is negative three. Let's look at the oxalates. Oxalate has a charge of negative two and that's multiplied by three oxalate ligands. So we know that the oxalate portion of this complex contributes a charge of negative six. Since our overall charge is negative three, then our cobalt charge must be positive three. So let's look at the electron configuration of Cobalt, so elemental Cobalt, the closest noble gasses are gone. So we have a R and brackets and then it is four S two 3d 7. So therefore, Cobalt three plus, which has lost three electrons. We still have our argon in brackets and it loses the 24 S electrons and one of the 3d electrons and we're left with 3d 6. So we have six D electrons to fill in our orbitals. Well, now we need to look at what our geometry is to determine what our levels will be. We know that Cobalt cobalt, excuse me, we know that oxalate is a bent ligand and we have three of them. So three multiplied by a bent is equal to three multiplied by two bonds per ligand equals six bonds. So we have an octahedral geometry. So with octahedral geometries, we know our elect our orbital levels. We will have two higher energy orbitals and three lower energy D orbitals. Now, we need to determine what's the size of the delta between them. How do they fill up? Well, a problem tells us that oxalate is a weak filled ligand, which means there is a small splitting. So this is a small delta. So as we fill in our 60 electrons, we will fill in one electron in each of the five D orbitals before we start pairing them up. So we have 60 electrons, we fill in one, each in the five D orbitals. And then the last one goes down below, we've got one pair. So we end up with a total of four unpaired electrons. So we have our crystal field energy level diagram, two single electrons and two higher energy levels or energy level orbitals, one pair and two singles in the three lower energy level orbitals and four unpaired electrons. See you in the next video.