What are Coordination Compounds?
The compounds in which ligands (electron pair donating species) are attached to transition metal through coordinate covalent bonds are called coordination compounds or complexes.
Transition elements have great tendency to form coordination compounds or complexes. A compound which has one or more coordinate covalent bonds is called coordination compound or complex.
Transition metals due to having ability of forming vacant hybrid orbitals are capable of forming complexes. Ligands donate their pairs of electrons to the vacant hybrid orbitals and make coordinate covalent bonds.
Transition elements have the tendency to form coordination compounds or coordination complexes due to high positive charge on transition metal cation and also due to the presence of vacant hybrid orbitals with them.
Fe → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6
Fe+2 → 1s2, 2s2, 2p6, 3s2, 3p6, 4s0, 3d6
Due to strong interaction with ligands the unpaired d electrons are paired up and as a result two orbitals of d sub-shell become vacant. Two vacant d-orbitals, one vacant s-orbital and three vacant p-orbitals get hybridized and give six vacant d2sp3 hybridized orbitals.
Ligands donate their lone pairs of electrons to the vacant hybrid orbitals of transition metal and thus form coordinate covalent bonds with central transition metal and as a result complex is formed.
Components of complexes Or the terms used in complexes:
Different terms used in study of complexes are as follow:
- Central metal atom or ion:
Transition metal atom or ion to which ligands are attached due to coordinate covalent bonds is called central metal atom or ion respectively.
e.g.(i) In K4[Fe(CN)6], Fe is central metal ion
e.g. (ii) In [Ni(CO)4], Ni is central metal atom
The atoms or molecules or anions which make coordinate covalent bonds with central transition metal by donating electron pairs(s) to transition metal, are called ligands. The ligands are electron pair donor while transition metal is electron acceptor.
Ligands could be anionic, cationic or neutral.
e.g.(i) In K4[Fe(CN)6], CN- is anionic ligand.
e.g. (ii) In [Ni(CO)4], CO is neutral ligand.
Ligands could be classified on the basis of number of electron pairs which they donate to the central transition metal, into three types.
- Monodentate Ligands:
Ligands which donate only one lone pair of electrons to form coordinate covalent bond with central transition metal are called monodentate ligands
e.g Cl-, Br-, NO2-,OH-, CN-, RCOO-, CO, NH3, H2O
- Bidentate Ligands:
Ligands which donate two lone pairs of electrons to form coordinate covalent bond with central transition metal are called bidentate ligands.
- Polydentate Ligands:
Ligands which donate more than two lone pairs of electrons to form coordinate covalent bond with central transition metal are called polydentate ligands.
e.g. Ethylene diamine tetra acetic acid (EDTA) ion. It is a hexadentate ligand.
- Coordination Number:
Number of monodentate ligands attached to central transition metal or number of coordinate covalent bonds that are made by transition metal, is called coordination number. Known coordination numbers are 2,3,4,5,6,7,8 and 9.
In K4[Fe(CN)6], coordination number of Fe is 6.
In [Ni(CO)4], coordination number of Ni is 4.
Complexes with 4 and 6 coordination number are very common while complexes with coordination number 5 are less common.
Ag usually shows coordination number 2
Ni, Cu, Pt usually show coordination number 4
Fe, Co usually show coordination number 6
- Coordination Sphere:
The central metal atom along with ligands is called coordination sphere. It is placed in square brackets. It may be anionic, cationic or neutral e.g. K4[Fe(CN)6], [Cu(NH3)4]SO4 and [Ni(CO)4] are anionic, cationic and neutral respectively.
- Charge on Coordination Sphere:
The charge on the coordination sphere is the algebraic sum of the charges present on the central metal ion and ligands e.g. In [Fe(CN)6]-4 the charge on Fe is +2 and that on each CN is -1, so the charge on coordination sphere is (+2) + (-1 x 6) = + 2 -6 = – 4.
A complex in which all donor atoms of a bidentate or polydentate ligand get coordinated with the same metal ion such that the complex formed contain one or more rings, such a complex is called chelate.
Bisoxalato platinate (II) ion Bisethylenediamine copper (II) ion
IUPAC names of coordination compounds follow following sequence
Cation – ligand – transition metal – anion
- Name of cation is written first if there is any cation.
(ii) Then name of ligand is written. Prefixes di, tri, tetra, penta are used with the name of monodentate ligand and prefixes bis, tris, tetrakis etc are used with the name of bidentate or polydentate ligands to show the number of ligands. If ligand is anion suffix “O” and for cationic ligand suffix “ium” is used with its name, while no special suffix for neutral ligand just its Latin name is written.
If more than one kind of ligands are there then first the negative, then the neutral and at the end the positive ligands are written. If more than one kind of –ve ligands are there then alphabetical order is followed among themselves, Same practice will be followed for +ve and neutral ligands if they are of more than one kind.
Prefix μ is used with the names of bridging ligands and they are written after normal ligands.
NH3 Ammine , H2O Aqua
CO Carbonyl , NO Nitrosyl
H2N – CH2 – CH2 – NH2 Ethylene diamine (en)
Hydrazinium N2H5+ or (:NH2—+NH3), Nitrosonium NO+, Pyrazinium
(iii) Now name of transition metal is written.
If coordination sphere is anion then suffix “ate” is used with the Latin name of transition metal, otherwise just English name is written.
(iv) Now followed the name of transition metal, oxidation state of the metal is written in Roman numeral in parenthesis.
(v) Anion is written at the end if any.
- K4[Fe(CN)6] Potassium hexacyano ferrate (II) (ii) K3[Fe(CN)6] Potassium hexacyano ferrate (III)
Diammine silver (I) chloride (iv) Na3[Ag(S2O3)2]
Sodium dithiosulphato argentate (I)
(vi) Na3[Co (NO2)6]
Sodium hexanitro cobaltate (III)
Dichloro aqua chromium (III) nitrate
(viii) K3[Al(C2O4)3] Potassium trisoxalato aluminate (III)
(ix) [PtCl(NO2)(NH3)4]SO4Chloro nitro tetraammine platinum (IV) sulphate
Dichloro tetraammine Cobalt (III) chloride
Geometries of Coordination Compounds: Coordination compounds have definite geometrical shapes because the coordinate covalent bonds formed between ligands and central transition metal is directional. Hence, the ligands are arranged around central metal atom in clear spatial geometries.
The geometry depends on type of hybridization shown by central transition metal.
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