Transition Metal Chemistry 1
TRANSITION METAL CHEMISTRY (chm302)
The term Transition metal chemistry is first used by Mendeleev to describe the group 8 triads such as iron, cobalt and nickel. This is because in this periodic table some of their properties resemble that metals such as Chromium and Manganese that proceeded them, in that same role and some properties resemble elements like copper which follows at the beginning of the1st row. The triads are seen as a gradual transition btw the elements at the end of one row and those at the beginning of the next rows
In the modern form of the periodic table , Mendeleev's term transition elements has been taken up with an enlarged meaning . The elements from period four which separates typical elements from the left from those on the right are called TRANSITION ELEMENTS.
Thus, historically transition elements are those elements in the periodic table which bridge the gap btw the electropositive group 1 and 2 and electronegative non-metals of groups 13 to 17.
Based on this definition, these elements have some common properties
1. They are metals with good thermal and electrical conductivities
2. They have high melting and boiling points
3. They have and electropositive character towards aqueous solution of non oxidizing acids
We may take these set general on the basis of these broad definitions . The first 2 elements in the Zn sub group and Cd can be included . However it will be difficult to call Hg a transition element as it is a liquid at room temperature and has a relatively low boiling point and exhibit no electropositive character
transition elements can also be defined based on the electronic configuration of these elements.
According to this definition, transition elements are defined as the elements whose atoms or ions have a partially filled "d" or "f" orbital.
Based on this definition some characteristics of transition elements (T.M) are :
1. formation of colored compounds
3. variable oxidation states
4. tendency to form complexes
IUPAC defines a transition element has an element whose atom has an incomplete d sub shell or which gives rise to cations with an incomplete d sub shell. This definition excludes Zn,Cd and Hg from the the transition elements since they have a d-10 configuration.
They do form ions with +2 oxidation states but retains the d-10 configuration. Thus this definition corresponds to groups 3-11 on the periodic table (PT)
Scandium has the electronic structure [Ar] 3d1 4s2. When it forms ions it always looses the 3 outer electrons and ends up with an [Ar] structure. The scandium 3 ions has no d electrons . Hence it is not a transition element. The coinage metals, Cu, Au, Ag since cu2+ has a 3d9 configuration, Ag2+ has a 4d9 and Au3+ a 5d8 configuration. They have a partially filled orbital. So they are transition metals.
Transition elements are those in the d block of the PT. This definition emphasizes the 4 blocks of elements in the periodic table (PT). This definition is rather unsatisfactory because it leads to the inclusion of Sc and Zn as transition elements.
Reason is that :
1. they have only 1 oxidation state in their compound (Scandium 3+ and zinc 2+) whereas others have 2 or more.
2. there compounds are usually white unlike compounds of othe transition metals that are generally colored.
3. they show little catalytic activity
The transition elements may strictly be defined as elements having partly filled d or f shells in any of their commonly oxidation states. consequently the coinage metals, cu Ag, Au are treated as transition metals because they have partially filled d orbitals
1. They are all metals
2. They are practically all hard strong, high melting and boiling elements.
3. They form alloys with one another and other metallic elements
4. Many of them dissolves in mineral acids while few are unaffected by simple acids
5. with few exceptions they exhibit variable oxidation states.
6. They form colored ions and compounds in 1 if not all oxidation states
7. They form some paramagnetic compounds
They are about 56 transition elements that is subdivided into 3 main groups
a) the main transition elements or d block elements (DBE)
b) the lanthanides elements
c) the actinides elements
The main transition group or d block include those that correspond to the filling of the 3d, 4d and 5d respectively, the elements SC,Ti,V,Cr,Mn,Fe,Co,Ni,Cu, all have potentially filled 3d shells either in their ground states or the free atoms, all except copper or in one or more of the chemically important ions. All except Sc are called first transition elements.
The 2nd transition elements start with Y [Ar]4d15s2 others are Zr,Nb Mo,Tc,Ru,Rh,Bd,Ag.They all have partially filled 4d shells either in the free elements (all but Ag) or more of their chemically important ions (all but Y)
The third transition elements begins with Hf [Ar]6s25d2 and includes Ta,W,Re,Os,Ir,Pd,Au they have partially filled 5d shells in one or more chemically important oxidation states (OS) as well as (except Au) in the natural atom. The groups of the d block contains only three elements and corresponds to the filling of the 3d 4d and 5d shell respectively. In btw the 4d and the 5d levels is interposed first "f" level. The 4f shell which fills after lanthanides
Zn,Cd, and Hg form no compound in which the d shell is other than full that is they are fully filled d orbitals, Hence they are known transition elements metals are softer with low BP (boiling point) and Mp (melting point) because the d orbital is complete. Zn and Cd are considerably more electropositive than their neighbors in the transition groups lower than their ability to form complexes with NH3 amino,amide ions and Cyanide makes them similar to the other d group of the transition elements.
VARIABLE OXIDATION STATES OF THE 1ST TRANSITION ELEMENTS
The ability to exist in more than one oxidation states is the most likely features of the transition elements e.g. Fe3+ and Fe2+, Cu2+ and Cu+, Co2+ and Co3+ and Co4+ and Co5+. This is in contrast to the S block elements , where the number of valence electrons equals a group number and P block elements where the valence electrons either equals the group number 8 minus the group No. For the P block element inert pair effect is common.
The variable oxidation states exist because of the little difference in energy btw the (n-1)d and nS. i.e. the two orbital 3d and 4s,4d and 5s, 5d and 6s. Consequently the ionization energy for the removal of electron from thus orbital is quite similar. In the formation of compound, electrons are removed from both orbital are removed.
Elements Atomic no. E config Oxidation No.
Sc 21 3d1, 4s2 *+2 or +3
Ti 22 3d2, 4s2 +2,+3,+4
V 23 3d3, 4s2 +2,+3,+4,+5
Cr 24 3d5,4s2 +2,+3,+4,+5,+6
Mn 25 3d5, 4s2 +2,+3,+4,+5,+6,+7
Fe 26 3d6, 4s2 +2,+3,+4,+5,+6
Co 27 3d7,4s2 +2,+3,+4,*+5
Ni 28 3d8,4s2 +2,+3,+4
Cu 29 3d10,4s2 +1,+2,+3,
Zn 30 3d10,4s2 +2
*doubt about their existence
Certain pattern in oxidation states (OS) emerge across the period for transition elements. For instance, the No. of the OS of each ion increases up to Mn; once the d5 configuration is exceeded, the tendency of all the d electrons to participate in bonding decreases. Later transition metals have a stronger attraction btw protons and electrons. Hence more energy is required to remove electrons. Thus, Fe has a maximum OS of +6 though the 2nd and 3rd elements in this group attain a maximum OS of +8 in RuO4 and OsO4. The observe difference btw Ru, Os and Fe is attributed to the size increase.
Note: except in Cr and Cu no +1 OS exits and even then Cu +1 disproportionate (complex that give insoluble compound) except when complexing ions are present in solution .
i.e 2Cu ----------------> Cu+ + Cu
Cu ----------------> Cu+ ---------------> Cu2+
STABILITY OF THE VARIOUS OXIDATION STATES
Compounds are regarded as stable if they exist at room temperature and not oxidized by the air and not hydrolysed by water vapor and do not disproportinate or decompose at room temperature.
Within each of the transition group 3-12 there is a diference in stability of the various OS that exist.
generally the 2nd and 3rd transition series exhibit higher cordination no. and their high OS are more stable than the corresponding 1st transition series.
1. The elements Cu, Ag, and Au show OS of +1, +2, +3 however, the only simple hydrated ions found in solution are Cu2+ and Ag+. The univalent ions Cu+ and Au+ disproportionate in water consequently only exist as a soluble compound or complexes. It is doubtful if Cr+ exist except when stabilized in a complex
2. For all the elements, +2 and +3 states are known and are the most stable states. The +2 ions across the period start as a strong reducing agents (RA) and become more stable while the +3 ions start stable and become strong oxidizing agents (OA)
+2 more stable +3 more stable
3. When the elements are in lower OS they can be found as simple ions. However, transition metals in higher OS are usually bonded covalently to electronegative elements like O, N, F, Cl. The +3, +2 states exists as aqua ions due to the combination with the higher electronegative combinations
Give two definitions of THE and the relative merits and demerits of each.
On the basis of your answer to 1a justify why Zn, Cd, Hg, cannot be classified as transition metals.
Give 2 reasons to justify their classification as transition metals.
Explain why Eu exhibit only +2 and +3 OS but Uranium (Ur) forms compound in which the metal exhibit +3 to +6 oxidation states.
Answer to question 2
This is due to the difference in size. Ur is larger in size compared to Eu therefore; it is easier to remove electrons from Ur than Eu since the electrons of Ur are less bound to the nucleus compared to the electrons of the Eu atom
In +4 state
4 VO 2+ MnO2 TiO2+ oxo cations
5 V O2 +
6 CrO3 HcrO 4 - MnO4 2- CrO4 2- oxo cations
7 MnO4 -
In higher OS charge is high and size is small i.e. charge/radius ratio is very large. Consequently, polarizing power of the ion is very large. Thus ions readily form covalent ions with the oxygen to form oxo ions
In high OS these ions are highly acidic e.g. Mangenic acid H2MnO4nbut they are basic in low OS
There's a difference in number of OS from Mn to Zn. This is because the pairing of the electrons occur after Mn (Hund's rule) which in turn decreases the no. of available no. of unpaired electrons and hence the nos. of OS
The stability of higher OS decreases in moving from Sc to Zn. Mn and Fe6 are powerful OA and the higher OS of Co, Ni, Zn are unknown. The relative stability of +2 state with respect to higher OS particularly the +3 states increases in moving from left to right. This is justifiable since it will be increasingly difficult to remove the d electrons from the d orbital.
There's a tendency of intermediate OS to disproportionate
2Cu+ ------------------------->Cu + Cu2+
2Mn+6 ---------------------->Mn+4 + Mn+3
The lower OS are usually found in ionic compound and higher OS tend to be involved in covalent compound.
RELATIVE stability of OS is an extremely important topic in transition metal chemistry in is usually discussed in terms of the standard reduction potential E* values.
There are 3 main methods of summarizing the thermodynamic stabilities of OS of elements in aqueous solutions.
1. LATIMER DIAGRAMS
2. FROST DIAGRAMS OR VOLT EQUIVALENT
3. POURBAIX DIAGRAMS
1 and 2 are restricted to extremes of standard 1 molar H+ pH = 0 or standard OH- solutions pH= 14
3 expresses the variation in stabilities of OS as a function of pH btw values of 0 to 14 hence, more comprehensive.
In Latimer diagram is a list of the various OS of an element arranged in descending order from left to right with the appropriate standard reduction potentials placed btw each pair of states. The diagram for chromium in acid solution is written below.
Cr2O7 2- ----------------->Cr3+ --------------->Cr2+ --------------->
1.33v -0.41v -0.91v
MnO4 - ----->MnO4 2---->MnO4 3----->MnO2------>Mn3+------>Mn2+--->Mn
0.564v 0.274v 4.27 0.95 1.51 -1.18
1. Write a balanced half rxn for all of the redox steps shown in th latimer diagram
2. Determine the redox potential for MnO2 conversion to Mn3+ at pH =4
This reaction is in acidic medium
The standard redox potential E* and the uunbalanced redox couples rxns for elemental Mn, reduction and Tc in acidic medium are listed in the table below
Redox rxn E*
TcO2 -------->Tc2+ +0.60v
Tc2+ -------->Tc +0.40v
TcO4 - -------->TcO2 +0.74v
Use the info in the table to answer the following questions.
Construct latimer diagrams for the elements
TcO4 - -------->TcO2 -------->Tc2+ -------->Tc
0.74 0.60 0.40
The application of thermodynamics to inorganic solutions chemistry was introduced in the simplest type of diagram by Wendell Latimer. In Latimer diagram for an element, the value for the standard potential in volts is written over a horizontal line connecting species with the elements in difference OS. The most highly oxidized from of the element is on the left and the species to the right is successfully of lower OS.
The Latimer diagram for Mn in acidic phase is;
MnO4- ---->MnO4 2- --->MnO4 3- ---->MnO2 ---->Mn3+ ---->Mn2+ -
0.564v 0.274v 4.27v 0.95v 1.51
It is no crime if oxidation number is written under or above the specie
ClO4- -------->ClO3- -------->ClO2 -------->HClO -------->Cl2 -------->Cl-
+7 +5 +4 +1 0 -1
1.20 +1.18 -1.65 +1.63 1.36
In the Latimer diagram for chlorine in acidic medium