General Chemistry The Chemistry of The Transition Metals

Hi, welcome back to Educator.com.0000

Today's lesson from general chemistry is the chemistry of the transition metals.0002

We are first going to get introduced to what we mean by coordination compound and basically what they consist of.0010

After we get introduced to them, we are going to then turn our attention0017

to nomenclature which is learning the rules to name these coordination compounds.0021

Following this, we will then get into the geometries and structures that coordination compounds can commonly adapt.0027

Related to structures will be then something we call isomers.0036

Finally we are going to, in the last part of the lesson, go into0040

electronic structure of coordination compounds and basically what happens to the0045

valence electrons of a transitional metal when atoms or molecules become attached to them.0052

We are going to learn that that is called crystal field theory.0061

We will also get into what is called the spectral chemical series and the implications of the crystal field theory.0066

As always, we will finish up the lesson with a summary followed by some sample problems.0073

Coordination compounds is the term that refers to the following.0081

These are compounds that consist of a central metal atom or ion which have0085

attached a varying number of atoms or molecules via a coordinate covalent bond.0090

These atoms or molecules that are attached have their own names.0098

These are what we call ligands or ligands.0103

The term coordination compound can also be used interchangeably with the term coordination complex.0108

Basically a coordination compound can also have the central metal atom be electrically neutral or charged.0115

A case where we have a charged neutrality is when nickel has attached to it four carbonyl ligands.0122

We are going to learn the names of these shortly.0132

But here this is formerly nickel Ni⁰.0134

An example of a coordination compound where we have the transition metal to be charged, if it is FeCl₂ and 2NH₃s.0138

Here in this case, this is formerly Fe ²⁺.0149

What we are going to be focusing on in this session is coordination complexes where the central metal atom is actually a d-block element.0155

Let's go ahead and move on.0166

Now that we have been introduced to what we mean by a coordination compound, let's go ahead and move into how to name them.0168

This is what we call nomenclature of coordination compounds.0174

There is a lot of rules associated with this; let's get into it.0178

When writing formulas, you are first going to list the chemical formula or the chemical symbol of the metal first.0182

Followed by the ligand symbols in alphabetical order.0188

After this, if the complex is neutral overall, brackets are not needed.0194

However if the complex is an ion, you are going to enclose the entire complex in brackets followed by the charge as a subscript.0200

An example would be the following, Fe(CN)₆^4-.0214

This tells me that the entire complex ion here has an overall 4- charge.0225

But something like this, FeCl₂ followed by (NH₃)₂,0231

this tells me that the overall complex ion is neutral overall because you don't see brackets.0241

When the ligand is a molecule instead of an atom, sometimes the point of attachment0249

can also be emphasized in the order in which you list the letters of the molecule.0253

For example, we can say FeCl₂ and (OH₂)₂; this is basically water.0262

We are listing the oxygen first because it is the oxygen that is0271

attached to the iron center, not the hydrogens; because point of attachment.0279

There is more rules; when naming the complex... this is always harder.0298

You are going to list the ligand names first followed by the metal name.0302

A Roman numeral in parentheses indicates the oxidation number of the central metal ion.0306

If you have a neutral ligand, neutral ligands are always going to retain their full name with the exception of the following four.0314

H₂O is going to be now formally be called aqua.0324

NH₃ is called ammine; CO is called carbonyl; NO is called nitrosyl.0328

Anionic ligands will also change their suffixes.0336

If your anion such as chloride ends in ?ide, it is going to become with an ?ido suffix.0340

For example, chloride is going to become chlorido.0347

If something ends in ?ate, it becomes with an ?ato suffix.0354

For example, sulfate is going to become sulfato.0359

Finally if you have something ending in ?ite, it becomes ?ito.0365

An example of this will be nitrite anion becoming nitrito.0372

Furthermore if you have more than one of the same ligand, we are going to0380

use Greek prefixes to indicate the number of each ligand, omitting mono.0384

We are going to use di- for two, tri- for three, tetra- for four, etc.0390

Next sometimes the ligand name already contains a Greek prefix.0401

If that happens, we are going to use the following system.0408

Bis- is going to be replacing di-; tris- is going to replace tri-.0412

Tetrakis- is going to replace tetra-.0418

The other scenario where you have to use these unique prefixes is when the ligand is polydentate.0421

What a polydentate ligand is that it has more than one point of attachment to the metal.0429

Once again a polydentate ligand means it has more than one point of attachment to the metal.0446

We are going to go ahead and look at more examples of those in greater detail later on in this lecture.0451

Finally the final four rows, we are going to list the ligand names in alphabetical order ignoring any prefixes.0459

If the complex ion is anionic, the suffix ?ate is added to the metal ion name.0466

An exception is iron; you don't call it ironate.0472

But instead we are going to use I believe the Latin phrase, ferrate.0475

If the complex ion is anionic, you are going to list the cation name first.0481

If the complex ion is cationic, you are going to show the anion name last.0486

Let's go ahead and look at an example.0491

Let's take the following, NH₄Pt in brackets Cl₃NH₃.0495

In this case, my cation is ammonium.0504

My anion is the entire complex which means we don't call it platinum.0509

But we call it platinate.0517

We know that the ammonium is a 1+ overall charge.0520

We know that each chlorine is 1- each, giving me 3- overall.0527

We know that ammonia is neutral ligand overall.0535

This tells us that the platinum must have an oxidation state of 2+.0538

Now that we have established that, we can go ahead and name the compound.0546

This is going to be ammonium.0549

It is the name of the cation followed by the following.0554

We list the ligands in alphabetical order--ammonium ammine and then trichlorito-platinate(II).0556

Let's go ahead and do one more example for nomenclature.0572

This one is going to have in brackets chromium with four ammines attached and with two OH groups attached and then bromine.0579

In this case, bromine is our anion; this entire complex ion is our cation.0594

We know that amine is 0 overall; we know that hydroxo is 1- overall.0602

We know that bromine is 1- overall, telling us that chromium must be 3+ here.0608

Let's go ahead and name it; list the ligands in alphabetical order.0617

This is going to be tetra-amine followed by di...0621

Now hydroxide becomes renamed as hydroxito; then chromium(III) bromide which is the complete name.0633

That is nomenclature for coordination compounds.0648

Let's now move on to the structures of coordination compounds--what do they really look like.0653

We define the coordination number, which is also known as steric number, to be0658

the total number of points of attachment to the central metal atom or ion.0663

When I mean total points of attachment, I mean from all of the ligands.0671

The most commonly observed coordination numbers for these transition metal complexes is the coordination number 4.0675

When we have a coordination number of 4, we can get two common geometries.0684

Number one--tetrahedral; or number two--square planar.0688

In tetrahedron case, you have your central metal atom.0692

Then with the four ligands attached like so in a traditional tetrahedral format.0695

Of course for square planar, your central metal atom.0702

Now the ligands are arranged 90 degrees to each other, giving you square planar overall.0706

The other common coordination number is coordination number of 6.0716

For coordination number of 6, we are going to basically get the main geometry of octahedral geometry.0721

Here you have your central metal atom.0730

You have the four ligands forming a square base.0733

Then you have one ligand perpendicular to that; another ligand perpendicular right below.0739

We call the four ligand positions that are horizontal, that is formed as square plane, we call them equatorial positions.0746

Of course then for the ligands that are perpendicular to that plane, these are called the axial positions.0763

Again tetrahedral and square planar are for a coordination number of 4.0777

Octahedral is for a coordination number of 6.0780

Now related to structures of coordination compounds, we are now going to discuss isomers.0784

Recall that isomers are basically the following.0792

These are compounds that have the exact same formula but have different chemical and/or physical properties.0795

Specifically for the coordination number of 6, we can have a large number of different isomers.0802

The first type of isomers we are going to talk about is for coordination number 6.0810

These are what are called geometrical isomers.0815

When we have a geometrical isomer, these are two formulas that you want to look for.0818

ML₄L'₂ and ML₃L'₃ where L and L' are different ligands.0823

When we look at ML₄L'₂, we can have one possible configuration0842

where the four Ls that are identical to each other,0847

they form the square plane like so--L' here and L' here.0851

In the other possibility, let's switch an L' with an L; good.0860

When the L's are completely opposite to each other, 180 degrees apart, this is what we call the trans isomer.0877

However when the two Ls here and here are not 180 degrees apart, when they are0888

perpendicular to each other like this, that is what we call the cis isomer.0897

Again this is what we call trans and cis isomerism.0904

The other type of geometrical isomer that exists for a coordination number of 6 is ML₃L'₃.0908

In this case, we could have the following two possibilities.0916

M and then L', L', L'... L, L, and L.0922

The other possibility, let's substitute an L' for an L.0941

In the left scenario, you see that the L's actually form a plane with each other like so that bisects the octahedron.0958

This is what we call the mer isomer.0973

Mer stands for meridian such that the three identical ligands share a plane that forms a meridian through the compound.0977

The other possibility on the right is when the three ligands are actually sharing the same face of the octahedron.0990

That is what we call the fac isomer, fac being short for facial.1000

That is geometrical isomers.1009

Let's now take a look at the category of what we call structural isomers.1012

When we talk about the word structural isomers, this is a very general category.1020

Basically structural isomers are going to retain a ligand position.1032

Excuse me... they are going to change ligand position for structural isomers; change ligand position.1049

There are basically four types of structural isomers that we want to deal with.1057

One is what we call ionization isomers; two is what we call hydrate isomers.1062

Three is what we call linkage isomers; finally four is what we call coordination isomers.1072

Ionization isomers literally have a cation or an anion switch places in or out of the brackets of the coordination sphere.1087

That is, in one isomer, a certain ligand is attached to the metal.1120

In the second isomer, it is not.1125

Let's go ahead and look at an example.1127

Imagine here in brackets, cobalt, bromine, and then five amines attached, closed brackets, and then sulfate.1130

We can imagine a situation where the sulfate just substitutes for the bromine.1140

We get CO(NH₃)₅SO₄ and then bromine.1146

My compound is still neutral overall because this is a salt after all.1155

The only difference is the location of sulfate with bromine.1162

The second type of structural isomer is what we call a hydrate isomer.1167

This simply involves ligand exchange with water.1171

Imagine chromium with three chlorines attached, coordinated to six waters.1184

We can imagine a situation where we have the waters now directly bonded to the chromium metal.1193

The three chlorines serving as counterions.1204

Linkage isomers, linkage isomers involve what are called ambidentate ligands.1210

They involve ambidentate ligands.1220

Basically the word ambidentate means the ligand can attach in more than one location to the metal.1225

Again the ligand can attach in different locations to the metal.1239

You could imagine a situation where you have transition metal; let's take a NO₂ group.1247

But you could also imagine the ligand being directly attached to the oxygen instead of the nitrogen.1257

In this case, we get a pair of linkage isomers.1263

Finally we can have also a fourth type of structural isomer called coordination isomers.1267

Basically you have two metals switching location with each other.1275

An example would be a chromium with the six amines attached followed by iron with the six cyano groups attached closed brackets.1292

That will be one coordination compound.1308

But you can also have another one where you just switch places.1310

Now iron is going to get the six amines.1313

Chromium is going to get the cyanos just like that.1318

Again these are what are known as structural isomers where we actually involve a change in attachment to the metal.1326

Now that we have covered isomers, let's go ahead and take a look at how the electronic structure is in coordination compounds.1336

To examine how the d-block electrons of the central metal atom or ion are1345

arranged once ligands are attached, we use something called crystal field theory.1349

Crystal field theory treats the lone pairs on a ligand as point negative charges.1356

These negative charges influence of course how the metal d-orbitals are arranged.1362

Let's go ahead and examine the most common type of geometry which is an octahedral field and a tetrahedral field.1367

For an octahedral field, you can imagine the following.1375

Your transition metal in the middle; then the ligands are on the outside.1380

These are all treated again as point negative charges like that.1384

Here is the octahedral framework.1392

It is determined that of the five metal d-orbitals, two get raised in energy and three go down in energy.1400

It turns out that d_xy, the d_xz, and the d_yz orbitals lie in between the ligands.1413

Because they lie in between the ligands, there is no electron-electron repulsion.1430

Their energies go down.1440

The two remaining d-orbitals, d_x2-y2 and d_z2, are found to lie directly pointing toward the ligand positions.1445

Because they lie directly toward the ligand positions, there is electron-electron repulsion occurring.1462

Because of this, their energies go up.1470

You can imagine the five d metal orbitals.1479

All of a sudden, once the ligands attach, you get two going up and two going downward.1482

The two metal orbitals that are together, the d_z2 and the d_x2-y2, these are what we call the e_g orbitals.1490

The three on the bottom, the d_xy, the d_xz, and the d_yz, these are what we call the t_2g orbitals.1499

The difference in energy between the e_g and the t_2g levels are what we call Δ_o.1506

Δ_o is what is known as the CFSE.1512

The CFSE is what we call the crystal field splitting energy.1517

Now that we have look at an octahedral field and we know how the d orbitals are what we call split,1532

let's now take a look at electron configuration using crystal field theory.1538

Consider a vanadium(II) ion in an octahedral field.1543

Vanadium has the configuration... vanadium 2+ is going to have the configuration argon followed by 3d³.1550

We are only dealing with 3d electrons here.1560

Because this is an octahedral field, you get the following 2-3 split.1563

Just as we have learned how to fill orbitals using Aufbau principle, we are still going to follow that.1570

d³, with the first d electron, second, and the third.1577

For d³, this is the only possible ground state configuration.1582

Once again this is the only possible ground state configuration.1590

However something different happens when we consider a chromium(III) ion in an octahedral field.1598

The configuration for chromium 3+ is going to be argon 3d⁴.1608

Now we actually have more than one possible ground state configuration because I can do one, two, three, four.1617

But also I can have one, two, three, and four.1629

Basically is the configuration going to be t_2g⁴?1637

Or is the configuration going to be t_2g³ e_g¹.1644

What that depends on is the magnitude of the crystal field splitting energy.1651

It depends on Δ_o.1659

Basically if Δ_o is too large, then we are going to get the t_2g⁴.1662

If Δ_o is small enough, that is negligible enough, then yes1674

it will be relatively easy to promote that last electron to the upper levels.1681

We will get the t_2g³ e_g¹ configuration.1686

The situation where Δ_o is too large, this is what we call the low spin scenario.1693

The situation where Δ_o is very small is what we call the high spin scenario.1702

How do you determine if Δ_o is small or large?1713

That is strictly going to be dependent on the ligands.1717

Let's go ahead and take a look at a series of common ligands next.1722

Here we have what is known as the spectral chemical series.1727

The spectral chemical series lists commonly encountered ligands in coordination chemistry starting with the carbonyl and cyano ligands.1736

Wrapping it up on the other end of the spectrum are the halides.1750

The way to read this is the following.1754

Ligands at the leftmost end are called strong field ligands because they result in a large crystal field splitting energy.1756

As a result, with these ligands, we get low spin configurations.1764

Ligands at the rightmost end are called weak field ligands because they result in a small crystal field splitting energy.1769

As a result, we often get high spin configurations with these ligands.1778

What you have to be on the lookout for are both possible high and low spin scenarios.1783

These will arise for configurations d⁴, d⁵, d⁶, and d⁷.1792

What is the big deal?--you may be wondering; why bother with this?1800

Now let's go ahead and examine the implications.1803

This is where it gets very interesting; the implications are the following.1805

We can use crystal field splitting theory to help us predict both the color and the magnetic properties of coordination compounds.1810

Let's first compare the magnetic properties of the following.1820

Iron with six aqua ligands attached making this 2+ versus now iron with six cyano ligands attached making that 4-.1824

In this situation, this is Fe²⁺.1841

In this situation on the right, this is also Fe²⁺.1844

Both of these complexes are d⁶.1848

The only difference obviously then is the identity of the ligand.1853

Because here H₂O, octahedral d⁶; here we are going to have cyano d⁶.1858

Let's start with the extreme.1867

Cyano is one of the first ligands you encounter.1868

It is one of the strongest field ligands.1871

Because it is strong field, that means it is going to be low spin.1874

One, two, three, four, five, six.1878

You notice that all of my valence electrons are paired.1882

For the cyano compound, we expect diamagnetism.1886

However for the aqua compound, it is determined that it is actually high spin.1893

One, two, three, four, five, six.1898

You see that we get plenty of unpaired electrons.1901

For the aqua complex, we expect paramagnetism.1904

You see how useful crystal field theory can be in explaining for the magnetic behavior of certain coordination complexes.1909

Now a brief discussion on color.1919

We want to review what is known as complimentary colors.1922

Complimentary colors is coming really from physics.1930

This tells us that the color of light that an object absorbs will be complimentary1934

to the color of light that is transmitted to our eyes, basically what you and I see.1940

You have tried to construct something called a color wheel like so.1947

Basically complimentary colors are going to be opposite to each other.1957

For example, if something is going to absorb orange color light, something is going to look blue to us.1961

How does this relate to crystal field theory?1972

Basically that crystal field splitting energy Δ_o is going to be proportional1976

to the energy of the photon absorbed which means that it is1984

complimentary to the energy of the photon transmitted or given off1995

or the photon that travels to our eyes, what you and I see.2010

If Δ_o is large, a weaker color of light is going to be given off.2016

If Δ_o is small, a stronger color of light is going to be given off.2034

If Δ_o is very large, you basically have a good bet of getting something like red, orange, or yellow.2047

If Δ_o is small enough, the object will most likely look violet blue or green to us or any combination thereof.2055

Once again using crystal field theory, we are able to explain for both the color and magnetic properties of coordination compounds.2067

This is very fascinating material actually.2074

To summarize, coordination compounds contain a central metal atom or ion attached to ligands which may be atoms or molecules themselves.2079

Coordination compounds are often observed with coordination numbers of 4 or 6, giving tetrahedral, square planar, and octahedral geometries.2089

Finally as we just demonstrated, crystal field theory can explain for both the color and magnetism of coordination compounds.2097

Now let's go ahead and tackle some sample problems; name the following compounds.2108

Here we see that the coordination complex is a cation and that the chlorides are the anions.2115

We know hydroxide is 1-; we know that water is neutral.2129

We know that each chloride here is 1-.2135

This tells us that in order to cancel the overall charge of the chlorides, that iron here must be 3+.2139

Let's go ahead and name them; alphabetical order.2151

We are going to get aqua first; there is five of them.2153

That is going to be penta-aqua followed by hydroxide; but hydroxide becomes hydroxido.2156

Then followed by iron; we don't use ferrate because this is a cation coordination sphere.2166

Then chloride; so pentaaqua hydroxido-iron(III) chloride.2175

Now onto the bottom one; here we have also another one.2184

Your bracket is your cation; your sulfate of course is going to be your anion.2189

We know that ammine is 0; we know that water is 0.2196

Which means that in order to balance the charges here, we have three sulfates.2200

Actually this should only be one sulfate here... my apologies.2214

We know that sulfate here is 2-; the cobalt here must be 2+ overall.2222

Let's go ahead and name this compound.2228

This is going to be triamine triaqua cobalt(II) and then sulfate.2230

That is sample problem one, focusing on nomenclature of coordination compounds.2244

The last sample problem deals with magnetism.2252

What change of magnetic properties if any can be expected when NO₂^1- ligands2255

in an octahedral field are replaced by chloride ligands in a d⁶ complex?2261

When we look up the spectral chemical series, we see that the NO₂^1- ligand is going to be a stronger field ligand than Cl^1-.2266

This is going to give us a larger Δ_o.2279

This is going to give us the smaller Δ_o.2283

Remember large Δ_o means it is going to be low spin.2287

Small Δ_o means it is going to be high spin.2294

Let's go ahead and draw the possibilities out.2298

d⁶ low spin means that all of my electrons are paired.2301

That is going to be diamagnetic.2308

Here we have d⁶ high spin.2310

All of a sudden, we get at least one unpaired electron.2315

That is going to be paramagnetic.2319

To answer this question, when you replace the NO₂^1- ligands with the chloride ligands,2321

you go from a compound that is diamagnetic to one that exhibits paramagnetism.2327

That is our lecture today on the transition elements.2335

I want to thank you for your time.2339

I will see you next time on Educator.com.2340