Chemistry of d - block elements is very interesting. Pink coloured KMnO4, orange coloured K₂Cr₂O₇, Mercury in thermometers, alloys like brass and bronze all the d-block elements. Crackers are also made of d-block elements to celebrate colourful Diwali. Fe, Cu, Ag, Au played an important role in the development of human civilization. Th, Pa & U are f-block elements, good sources of nuclear energy.
            These elements are placed between s & p block elements in the periodic table. The word 'd-block' means, the differentiating electron enters into d-subshell and the elements are called "d-block" elements (groups 3 to 12). Where as the elements containing atleast one unpaired 'd' electron are called transition elements (groups 3 to 11). Both the types have (n - 1)d^{1 - 10} ns^{1 - 2} configuration.
            There are four series of the d-block elements 3^d series (Sc to Zn), 4^d series (Y to Cd), 5^d series (La to Hg excluding Ce to Lu) and 6^d series (Ac to Cn excluding Th to Lr). 4^f series (Lanthanides) and 5^f series (Actinoides) are called inner transition elements.
           Due to small atomic size than that of 's' block elements, high nuclear charge, presence of unpaired 'd' electrons, these elements exhibit typical characteristic properties.

ATOMIC AND IONIC RADII
           Atomic and ionic radii of the d-block elements decrease with increase in atomic number upto Cr group and later remains constant due to increase in shielding effect. Ionic radii decreases with the increase in positive oxidation state (Fe⁺² > Fe⁺³).
IONIZATION ENTHALPIES
         Ionization enthalpies increases (some irregularaties at d⁰, d⁵, d¹⁰) from left to right in each series. Ionization enthalpies give an idea of stability of oxidation states.

OXIDATION STATES
         As there is a small energy difference between ns and (n - 1)d orbitals, both the energy levels take part in bonding and exhibits variable oxidation states (except Sc). Upto 7th group maximum oxidation state of an element is sum of s and d electrons. Cr group elements has lowest +1 (No. of s electrons, oxidation state). Common oxidation state of d-block elements is +2. Among 3d series maximum oxidation state of Mn is +7. Among 4^d & 5^d series Ru & Os is +8. Oxidising & reducing behaviour depends upon oxidation states. Cr⁺² is reducing as its configuration changes from d⁴ to d³, Mn⁺³ is oxidising as its configuration changes from d⁴ to d⁵. Sometimes same element in a reaction exhibits 3 different oxidation states, among them one oxidation state is unstable (middle oxidation state) compared to 2 other oxidation states (lower & higher oxidation states) is called "disproportionation of oxidation state".
                            
           Mn⁺⁶ is less stable oxidation state, compared to Mn⁺⁷ & Mn⁺⁴. The standard electrode potentials [EM⁺²/M] could explain the ability or inability of an atom to liberate H₂ from acids.
ECu⁺²/Cu is + 0.34 V (+ve) due to high enthalpy of atomization and low enthalpy of hydration and Cu is unable to liberate H₂ from acids. S.E.P. values tells about the stability of ions like Mn⁺² (half filled 3d⁵) and Zn⁺² (completely filled 3d¹⁰).

MAGNETIC PROPERTIES
       Though many of the transition metals are paramagnetic, they will show diamagnetism and Ferro-magnetism. Para magnetism is due to unpaired 'd' electrons and attraction of the substance (B > H) by the magnetic fields (e.g.: Cr⁺³, Sc⁺², Fe⁺³, Mn⁺²). Diamagnetism is due to paired 'd' electrons and repulsion of the substance (B < H) by the magnetic field (e.g.: Zn⁺², Cu⁺¹, V⁺⁵, Ti⁺⁴). Ferromagnetism is due to presence of more number of unpaired 'd' electrons and strong attraction of the substance by the magnetic field (B > > > H) e.g.: Fe, Co, Ni. Each electron having magnetic moment associated with spin angular momentum and orbital angular momentum. But for the compounds of 3^d series of elements orbital angular momentum is negligible. The "spin-only" formula to calculate angular momentum.
                
              where n = no. of unpaired electrons.

COLOURED IONS
           Almost all the transition metal compounds are coloured in solid as well as aqueous state. Due to presence of unpaired 'd' electrons and d - d transition energy of excitation corresponds to the frequency of light (visible region) absorbed. Frequency of the light depends upon the nature of the ligand. The colour observed corresponds to the frequency of the complimentary colour of the light absorbed. Sc⁺³, Ti⁺⁴, V⁺⁵ are colourless (absence of unpaired 'd' electrons), Cu⁺² is blue coloured due to presence of one unpaired 'd' electron. The metal ions may exhibit different colours in different oxidation states. e.g.: Cr⁺² = blue, Cr⁺³ = green, Cr⁺⁶ = yellow, Cr₂O₇^-2, CrO₄^-2 and MnO₄- have d⁰ configuration, but all are coloured due to charge transfer phenomenon. CuSO₄ . 5 H₂O has 5 ligands (H₂O). These ligands cause splitting of d - orbitals &
d - d transition of the unpaired d - electron. So CuSO₄ . 5 H₂O is blue in colour where as anhydrous CuSO₄ has no ligands and is colourless.

CATALYTIC PROPERTIES
      Transition metals and their compounds have catalytic activity due to ability of giving variable oxidation states. V₂O₅ is used in Contact process, finely powdered Fe is used in Haber's process, Ni is used in making dalda. These elements become more effective by changing their oxidation states.
               e.g.: Fe⁺³ catalyses the reaction between I- & S₂O₈^-2.
                       2 Fe⁺³ + 2 I-  → 2 Fe^₊₂ + I₂
                       2 Fe+2 + S2O₈^-2→ 2 Fe+3 + 2 SO₄^-2

INTERSTITIAL COMOUNDS
          Non stoichiometric compounds formed when interstitial sites of crystal lattices of metals are occupied by small atoms like C, H, N etc.
e.g.: TiC, Mn₄N, Fe₃H, VH_0.56, TiH_1.7.
         They have high M.P.'s are hard (borides are hard like diamond), are chemically inert and good conductors of electricity.

Lanthanides: Lanthanides (4f) are silvery white soft metals, good conductors of heat and electricity, M.P.s, B.P.s, densities increase from La to Lu, are paramagnetic (except La⁺³, Ce⁺⁴, Yb⁺², Lu⁺³), have low ionisation enthalpy, chemically reactive and effected by air (to form M₂O₃ or MO₂) and H₂O (to form M(OH)₃). Their oxides and hydroxides are basic in nature. They are coloured. The colour of lanthanide (III) ion with 4fn configuration is similar to the ion with 4 f^{14 - n}
              e.g.: Nd⁺³ (4f₃) & Er⁺³ (4f^{14 - 3}) - Pink
                       Sm⁺³ (4f₅) & Dy⁺³ (4f^{14 - 5}) - Yellow.
             The gradual decrease in atomic radii, ionic radii (or size) of 14 Lanthanide elements due to poor shielding of valence electrons and diffused shape of f orbitals is known as "Lanthanide Contraction". The consequences of Lanthanide contraction are:
            Properties of 4^d & 5^d elements are similar (Zr & Hf have identical radii).
            It is difficult to separate Lanthanides from their mixture due to similar crystal structure and properties.
 Lanthanides behave like Calcium, with increasing atomic number they behave like Aluminium. Lanthanides are used to produce steel plates and pipes. Alloy MISCH metal consists 95% Lanthanide + 5% Iron + traces of S, C, Ca, Al is used to produce bullets. Mixed oxides of Lanthanides are used in petroleum cracking. Individual oxides are used in television screens (as phosphors).
            Actinides are radioactive and their study becomes more difficult. Actinide contraction is greater from element to element. Actinide contraction is greater from element to element than Lanthanide contraction as the 5^f electrons are more effectively shielded from nuclear charge. Meanwhile 5^f electrons provide poor shielding from element to element in the series.
             The common oxidation state of Actinides is +3. Np & Pu exhibit maximum +7 oxidation state. 'These elements are highly reactive with HCl, least reactive with HNO₃ due to formation of protective oxide layer, non reactive with alkalies.

Uses of d and f-block elements
* Iron & steel are important construction materials.
* TiO is used in pigment industry.
* MnO₂ is used in dry battery cells.
* Zn, Ni, Cd are used in battery industry.
* Cu, Ag, Au are coinage metals.
* UK copper coins are made of copper coated steel, silver coins are made of Cu, Ni alloy.
* Catalyst V₂O₅ is used in the manufacture of H₂SO₄. Zieglar - Natta catalyst used in the manufacture of polythene. Fe is used in the manufacture of NH₃ (Haber's process), Ni is used in hydrogenation of oils to get Dalda, PdCl₂ is used in Waker process to oxidise C₂H₄ to CH₃CHO, AgBr is used in photographic industry.

COORDINATION COMPOUNDS
             Coordination compounds (complexes) play important role in biological systems like Chlorophyll, Haemoglobin, Vitamin B-12, in metallurgy to extract Ag, Au, to purify Ni metals, in electroplating in Photography, in qualitative analysis to detect Ni+2, Co₊₂, Fe⁺³, Zn⁺², Cu⁺², Hg⁺² ions in Volumetric analysis (EDTA titrations) and in the estimation of hardness of water.
               Complex compounds are formed when the metal ion is coordinated with ligands. As transition metals have small atomic size, high ionic charge, vacant d orbitals they form complexes. The species (atoms/ molecules/ ions), which donate a pair of electrons to central metal atom or ion to form a dative bond are called ''ligands". Ligands may be ions (e.g.: Cl ^-, CN ^-) or neutral (NH₃, H₂O). Ligands are Lewis bases (electron pair donor) and the central metal atom or ion is Lewis acid (electron pair acceptor). If ligand contain one donor atom (NH₃, H₂O) is called unidentate, 2 or more donor atoms are called polydentate ligands. Oxalate ion or en are bidentates, EDTA is hexadentate. A unidentate ligand containing two donor atoms, can coordinate through either of donor atoms is called ambidentate ligand (e.g.: NO₂- coordinate through N or through oxygen in ONO).
The total number of unidentate ligands around central metal atom or ion in a complex is called "coordination number". If a central atom in a complex is bound by one kind of ligands, that complex is called "Homoleptic complex" (e.g.: [Co(NH3)6]⁺³). If a central atom in a complex is bound by two or more different types of ligands is called "Heteroleptic complex"
(e.g.: [Co(NH₃)4Cl₂]+). If the complex ion carries negative charge is called "anionic complex" (e.g.: K₄[Fe(CN)₆], positive charge is called "cationic complex" (e.g.: [Co(NH₃)6]Cl₃) and no charge is called "neutral complex" (e.g.: [Ni(CO)₄].

WERNER'S THEORY OF COORDINATION COMPOUNDS
           * Alfred Werner gave satisfactory explanation to the mechanism of formation of complex. Some of the important postulates are:
           * Every complex compound has central metal atom (or ion).
           * Central metal ion shows 2 types of valancies.
Primary valance:
           * It is equal to oxidation state of central metal ion.
           * It is ionisable valence.
 It is non directional & represented by .............. line.
           * It is satisfied by -ve ions only.
Secondary valance:
           * It is equal to coordination number of central metal ion.
           * It is satisfied by ligands.
           * It is non ionisable valence.
           * It is directional and represented by line.
           * It is satisfied by -ve ions as well as neutral molecules.
Defects: This theory could not explain colour and magnetic behaviour of complex compounds. This theory does not correlate electronic configuration of the central metal atom with the formation of the complex.

IUPAC NOMENCLATURE OF COORDINATION COMPOUNDS
Rules for writing formula:
* Formula of the cation (simple or complex) is written first followed by the anion.
* Coordination part is written in square bracket.
* The symbol of the metal is written first followed by symbols or formulae of ligands alphabetically according to their names. e.g.: [CO(NH₃)4 (H₂O) Cl] Cl₂ order of ligands is amine, aqua and chloro.
* The metal atom and ligands are written continuously without any gap between them
* In case of abbreviated ligands (e.g.: en for ethane 1, 2 - diamine), first letter of the abbreviation is used to determine the position of the ligand in the alphabetical order.
* If the formula of only the complex ion is written, charge of the ion is written on the right side of the bracket. e.g.: [CO(NH₃)6]⁺³.
Rules for writing name:
* Name of the cation is written first followed by the anion.
* Oxidation state of the central metal atom is written in bracket and shown by Roman number.
* Names of the negative ligands are ended with -O. e.g.: Cl ^- (Chloro), NO₂^- (Nitro), OH^- (hydroxo), CN^-(Cyano). According to 2004 IUPAC recommendations Cl ^- is named as chlorido, CN^- as Cyanido, H^- as hydrido (Old & new name).
* Names of the positive ligands are ended with -ium e.g.: NO⁺ (Nitrosonium), NO²⁺ (Nitronium).
* Neutral ligands have no special ending. e.g.: CO (Carbonyl), NH₃ (amine), H₂O (aqua present name, aquo old name).
* One word name is given for a neutral complex.
* Name of the metal remains same in neutral and cationic complexes.
* Name of the metal ends with -ate in the case of anionic complex. e.g.: Cr ^- Chromate, Fe ^- ferrate, Cu ^- Cuprate, Co ^- Cobaltate.

Structural Isomerism:
           The isomerism that arises due to the difference in structure (due to different

In octahedral complex, fac (same ligands occupy adjacent positions at the corners of tetrahedron) and mer (same ligands occupy around the meridian of the octahedron) isomers are possible.

If the light is rotated towards right side is called "dextro" isomer, towards left side is called "laevo" isomer.

Bonding in coordination compounds
           As Werner's theory could not answer directional nature of the bonds, formation of coordination compounds by certain elements, magnetic & optical properties, many theories like VBT, CFT, LFT, MOT came to explain these properties.
Valence Bond Theory
             This theory was extended to complex compounds by Linus Pauling to understand valencies and their structures.According to this theory, metals lose electrons to form cations. Number of electrons lost is equal to its oxidation state.Vacant orbitals undergo hybridization to give shape to the complex. Hybridized orbitals occupy electron pairs donated by ligands. The electrons of the orbitals may undergo regrouping. Orbitals of ligand overlap with vacant metal orbitals to form covalent bonds. The metal acquires the next inert gas (nearest to the inert gas) structure.
The central metal atom either uses inner orbitals i.e. (n-1) d orbitals to form inner orbital complexes or outer orbitals (nd orbitals) to form outer orbital complexes.
Inner Orbital Complexes: These complexes are also called hyper ligated or strong field or low spin or spin paired complexes. As electrons of the metal pair up, unpaired electrons decreases or compound become para or diamagnetic. CO, CN^-, NH₃ ligands are strong. The unpaired (n-1) d electrons are rearranged to take part in hybridization to form inner orbital complex.
           e.g.: Formation of Ferro Cyanide ion [(Fe(CN)₆)]^-4

 Formation of [(Fe(CN)₆]^-4

        This ion is Octahedral and diamagnetic (as all the electrons are paired).

Outer Orbital Complexes: These complexes are also called hypo ligated or weak
field or high spin complexes. As the complex has large number of unpaired electrons, the
configuration of metal remains unchanged. F^-, Cl^-, H₂O ligands are weak. Here nd electrons take part in hybridization.
In [Ni(CO)₄] complex even CO is strong ligand, as 4s electrons are rearranged to 3^d orbitals to give sp³ hybrid orbitals.
e.g.: Formation of [FeF₆]^-4

 sp³d₂ H.O.S of Fe⁺² :

     This ion is octahedral and paramagnetic (as unpaired electrons are present).

LIMITATIONS OF VBT
* It does not distinguish between weak & strong ligands.
* Hybridisation in octahedral & tetrahedral complexes can not be predicted without knowing no. of unpaired electrons.
* It dose not predict the shape of complexes with coordination number 4 (whether it is tetrahedral or square planar).
* It does not explain the colour of the complex compounds.
* It does not give quantitative interpretation of magnetic data, thermodynamic or kinetic stabilities of complex compounds.

Crystal Field Theory
          This theory considers metal - ligand bond is ionic. This theory helps in explaining stability, magnetic moment, colour and orbital separation energies. Ligands are treated as point charges. Five d orbitals of isolated gaseous atom (or ion) have same energy (degenerate). Degeneracy won't be observed when it becomes asymmetrical due to anionic ligands or dipolar molecules (e.g.: H₂O & NH₃) and causes splitting of orbitals into 2 sets (t_2g and e_g).

Crystal field splitting in octahedral coordination entities:
       In octahedral complex compounds, when d orbitals split, if d_x2 − _y2 & d_z2 orbitals point towards the axes along the direction of the ligand, energy will be increased (eg set) if d_xy, d_yz, d_zx orbitals directed between the axes, energy will be lowered (t_2g set). The energy of two eg orbitals will increase by  & three t_2g will decrase by .

Order of field strength (Spectrochemical series):
        I^-< Br^-< Cl- < N^-3< F^-< OH^-< C₂O₄^-2< H₂O < NH₃< CN^- < CO. If ∆₀ (crystal field splitting) < p (pairing energy), 4th e- enters into eg orbitals (t_2g3e_g¹). This happens when ligands are weak field ligands and forms high spin complexes (outer orbital complexes). If ∆₀ > p, 4th e- occupy t_2g orbital (t_2g4e_g⁰). This happens when ligands are strong and forms low spin complexes (inner orbital complexes).
     In tetrahedral complexes, d orbitals splitting is inverted and smaller . 

olour of the coordination compounds can be readily explained in terms of crystal field theory (C.F.T.). For example in [Ti(H₂O)6]⁺³, Ti⁺³ has 3_d¹ configuration and absorbs light energy whose frequencey corresponds to visible region and this electron excites from
t₂g¹eg⁰→ t₂g⁰eg¹. It absorbs green colour and transmits violet colour and appears violet. In absence of ligands crystal field splitting does not occur and complex is colourless. Anhydrous CuSO₄ is colourless where as CuSO₄. 5 H₂O is blue in colour (due to splitting of d orbitals by the ligands and d - d transition of unpaired d electron). As Cr⁺³ ions (d³) occupy octahedral sites of Al₂O₃ and Ruby stone is formed. As Cr⁺³ ions (d_³) occupy octahedral sites of Beryl (Be₃Al₂Si₆O₁₈) and Emerald appears green.
            In metal carbonyls, strong synergic bonding is observed. The metal - carbon d bond in metal carbonyls possesses both σ (ligand to metal) bond and Π (metal to ligand) bond. The stability of coordination compounds is measured in terms of K (stepwise stability constant) or β (overall stability constant). The reciprocal of β is called instability or dissociation constant (1/β).
 For example if K₁, K₂, K₃, K₄ are stepwise stability constants and β₄ is over all stability constant.
          β4 = K₁ × K₂ × K₃ × K₄
         
      If β₄ of Cu(NH₃)₄⁺² ion is 2.1 × 1013 then dissociation
      
    
Applications of coordination compounds:
* EDTA, DMG are used in qualitative & quantitative chemical analysis.
* Hardness of water is estimated by Na₂ EDTA.
* In the extraction of metals, purification of the metals.
* Chlorophyll contains Mg⁺², Haemoglobin contains Fe⁺², vitamin B₁₂ contains CO⁺³ ions.
* Wilkinson catalyst [(PH₃P)₃ Rh Cl] is used in the hydrogenation of alkenes.
* Solutions of the complexes are used in electroplating.
* AgBr is used in photography.
* Lead poisoning is treated by using EDTA.