Class Note for CHEM 242 at UMass(6)
Class Note for CHEM 242 at UMass(6)
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Date Created: 02/06/15
Lecture Experiment 5 Measuring Metal Magnetism The objectives of this experiment as stated on slide 1 of the Powerpoint lecture slides are l to measure the magnetic susceptibility of metal complexes 2 to determine the magnetic moment of each metal 3 to determine the electron con guration whether highspin or low spin of each metal and 4 to compare these results to the ligands crystal eld strength Recall slide 2 how to draw both the lowspin and highspin configuration for any metal in any oxidation state In the highspin con guration the electrons are rst dispersed into both the lowlying and highlying orbitals then any remaining are placed in the lowlying orbitals In the lowspin con guration the electrons are rst dispersed only in the lowlying orbitals lling the highlying orbitals only when the lowlying ones are full The magnetic susceptibilty slide 3 of a solid x is the degree of its attraction or repulsion to or from an external magnetic eld It can be measured in many ways and units as the molar magnetic susceptibility XM which we will use the paramagentic magnetic susceptibility xpm and others The magnitude of x depends on 1 the magnetic moment generated on each metal atom by electon spin 2 what is known as spinorbit coupling a factor which increases a metal s magnetic moment and 3 the alignment of the individual magnetic moments in a solid xpm or xfmo or others The magnetic moment slide 4 generated on each metal atom by electron spin is the magnetism arising from the spin of unpaired electrons on that metal It is known as Meg meaning the effective magnetic moment as without also measuring the alignment of the individual magnetic moments we cannot determine it exactly We will just refer to it as the magnetic moment of each metal atom The magnetic moment is proportional to the number of unpaired electrons on a metal It is given by the equation Mz g S S l where S is the total electron spin the number of unpaired electrons divided by 2 g is called the Lande factor or the gyromagnetic ratio It is a constant the magnetic spin contributed by one free electron Its value for a free electron is 200000 to many decimal places However when the electron is not free but located on an atom the gfactor changes slightly A carboncentered electron has a gfactor of 20000001 a titaniumcentered electron has a gfactor of l900000 and other electrons centered on other metals have slightly different g factors We will use the approximation that for all our complexes g is approximately equal to 2 So if we substitute that S number of unpaired electrons divided by 2 and that g is approximately equal to 2 we will come up with the equation that we will use for calculating the magnetic moment of our complexes MEf 2 quotI101 2 S 0 means spinonly which will be explained below The next component of x to be considered is spinorbit coupling slide 5 This actually contributes to the magnetic moment ueff and therefore contributes to x indirectly Spinorbit coupling can occur when magnetism is generated in a complex by the metal s unpaired electron spins coupling with the valence orbitals angular momentum given by the orbital s magnetic quantum number m This will increase the value of the magnetic moment and so increase the value of the compound s magnetic susceptibility In many transition metal complexes however the spinorbit coupling is quenched by the binding of ligands to the d orbitals The contribution of the spinorbit coupling to the magnetic moment of our complexes therefore approaches zero and we will ignore it or as is said use the spinonly magnetic moment and not the magnetic moment that takes account of any spinorbit coupling The last factor to be considered in measuring the value of the magnetic susceptibility is the ordering or alignment of the individual magnetic moments within the solid slide 6 The magnetic moments may align in different ways all of which may affect the magnetic susceptibility As we see from slide 6 the magnetic moments of the individual metal atoms may be randomly aligned giving rise to paramagetism in which a factor of xpm is added to the magnetic susceptibility Paramagnetic substances are attracted to external magnetic fields xpm may be weak to moderate it is the resultant of all the randomly dispersed individual magnetic moment vectors In our powdered complexes to be used in this lab xpm is considered to be weak and will we ignore its effect on the overall molar magnetic susceptibility Ferromagnetism is achieved when all the magnetic moments are aligned giving rise to a very strong resultant and a very large xfem This is the type of magnetism seen in iron and in some other magnetic metals Antiferromagnetism is achieved when the individual magnetic moments are nearly all aligned antiparallel and thus nearly cancel each other out This contributes to a very low magnetic susceptibility Although on slide 6 the alignment of magnetic moments are drawn as pure phases this does not always occur in bulk solids They may have regions of ferromagnetism and of paramagnetism or especially common regions of paramagentism and antiferromagnetism within the same actual solid When a metal in a complex has no unpaired electrons slide 7 it is said to be diamagnetic Since there is no contribution to magnetism provided by electron spin S 0 the magnetic moment is also zero The magnetic susceptibility also closely approaches zero as it is so strongly dependant on magnetic moment But the paired electrons give rise to a very slight repulsion to magnetic fields causing a very small xdia to arise This diamagnetic susceptibility is too small to be measured by our experimental apparatus and we will take the magnetic susceptibility of diamagnetic complexes to be zero It can be exploited however in many applications slide 7 and it the basis for such machines as highspeed trains Magnetic Susceptibility vs Temperature Magnetic susceptibility is very sensitive to changes in temperature The individual magnetic moments may become more ordered at low temperature or less ordered at hgih tempertaure causing xpm to change dramatically Or an alignment change may occur such as from ferromagnetic alignment to anitferromagnetic alignment Regions of the solid which are at one alignment may grow larger or change to another alignment altogether Note that ueff at lower temperatures may become different from the ueffsospinorbit coupling may arise or different magnetic interactions may appear as discussed above Many of the complexes that we will use have the formula MSO4 tzO slide 9 Many of these are actually aquo complexes such as FeSO4397HzO which is actually FeHzO5SO439HzO Whether coordinated directly to water or perhaps to the sulfate ion all the donor atoms in these complexes are oxygen This will become important when you compare the highspinlowspin properties of these compounds to their ligand field strengths In conclusion we will determine the magnetism of various transition metal complexes with which many properties of practical value can be found For example from knowing the magnetism of complexes we can determine their strengths as magnets for use in electronc applications number of unpaired electrons oxidation states ligand eld strengths and magnetic coupling of metals in such complexes as organic catalysts and bioinorganic reaction centers