1 Coordination Chemistry of Transition Elements for 3rd Class By Dr. Khalil K. Abid Department of

53 Pages·2017·2.94 MB·English

Coordination Chemistry of Transition Metals Dr. Khalil K. Abid

Coordination Chemistry of Transition Elements
for 3rd Class
By Dr. Khalil K. Abid
Department of Chemistry , College of Science
University of Mustansiriyah, Baghdad – IRAQ

Email: [email protected]; [email protected]

1
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid

Transition metals, are located in groups 3 through 11. They are called transition metals because
they transition between the highly reactive s block metals and the much less reactive metals of group
12 and the p block.

Horizontal trends in key atomic properties of the Period 4 elements
2
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid
• Atomic size decreases overall across a period. The d electrons fill inner orbitals , so they shield outer
electrons from the increasing nuclear charge very efficiently and the outer 4s electrons are not pulled
closer.
• Electronegativity usually increases across a period but the transition metals exhibit a relatively small
change in electrinegativity.
• IE increase relatively little because the inner 3d electrons shield efficiently and the outer 4s electron
1
experiences only a slightly higher effective nuclear charge.
• Ionic bonding is more prevalent for the lower oxidation states and covalent bonding is more prevalent
for the higher oxidation states. At room temperature TiCl is an ionic solid and TiCl is a molecular liquid
2 4
• In high oxidation states atoms have higher charge densities ⇒ polarize electron clouds of non-metals
⇒ covalent bonding
• The oxides become less basic as the oxidation state increases TiO is a weak base in water and TiO2 is
amphoteric.
• Most main group ionic compounds are colorless because the metal ion has a filled outer level; On the
contrary,
• Electrons in particular filled d-sublevels can absorb visible wavelengths and move to slightly higher
energy d-orbitals. Therefore, many transition metal compounds have striking colors. Exceptions occur
when d orbitals are empty or filled. Zn+2: [Ar] 3d10 and Sc+3 or Ti+4 [Ar] 3d0.
Transition Metals: The genaral properties of transition metals can be summerized as follow;
1 – Have typical metallic properties. 2 – Not as reactive as Grp. IA, IIA metals. 3 – Have high MP’s,
high BP’s, high density, and are hard and strong. 4 – Have 1 or 2 s electrons in valence shell.
5 – Differ in # d electrons in n-1 energy level. 6 – Exhibit multiple oxidation states.
Characteristics due to d electrons: Exhibit multiple oxidation states; Compounds typically have color;
Exhibit interesting magnetic properties; paramagnetism; ferromagnetism.
Transition metals act as Lewis acids forming complexes/complex ions as
Fe+3(aq) + 6CN-(aq)  Fe(CN) -3(aq)
6
Lewis acid Lewis base Complex ion
Complex contains central metal ion bonded to one or more molecules or anions.
Lewis acid = metal = center of coordination.
Lewis base = ligand = molecules/ions covalently bonded to metal in complex.

3
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid
Sc Ti V Cr Mn Fe Co Ni Cu Zn
+1 +1
+2 +2 +2 +2 +2 +2 +2 +2 +2
+3 +3 +3 +3 +3 +3 +3 +3 +3
+4 +4 +4 +4 +4 +4
+5 +5 +5 +5
+6 +6 +6
+7
Oxidation States of Transition Elements
Coordination compound: Compound that contains 1 or more complexes
Examples: [Co(NH ) ]Cl ; [Cu(NH ) ][PtCl ]
3 6 3 3 4 4
Coordination sphere: Metal and ligands bound to it
Coordination number: number of donor atoms bonded to the central metal atom or ion in the complex
Most common = 4, 6 determined by ligands
Larger ligands and those that transfer substantial negative charge to metal favor lower coordination
numbers. The molecules or ions coordinating to the metal are the ligands are usually anions or polar
molecules. They must have lone pairs to interact with metal.
Complex charge = sum of charges on the metal and the ligands. [Fe(CN) ]-3
6
+3 6(-1)
Neutral charge of coordination compound = sum of charges on metal, ligands, and counterbalancing
ions. [Co(NH ) ]Cl
3 6 2
+2 6(0) 2(-1)
Ligands: classified according to the number of donor atoms
Examples; monodentate = 1; bidentate = 2; tridentate = 3; tetradentate = 4; hexadentate = 6

Chelating agents
polydentate = 2 or more donor atoms
Monodentate: ex. : H O, CN-, NH , NO -, SCN-, OH-, X- (halides), CO,
2 3 2
Complexes; [Co(NH ) ]+3 , [Fe(SCN) ]-3
3 6 6
Bidentate: Examples; oxalate ion = C O -2 , ethylenediamine (en) = NH CH CH NH
2 4 2 2 2 2
Complexes; [Co(en) ]+3 , [Cr(C O ) ]-3
3 2 4 3
4
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid
Hexadentate: ethylenediaminetetraacetate (EDTA)
Complexes: [Fe(EDTA)]-1 , [Co(EDTA)]-1

O O

O C CH CH C O
2 2

N CH CH N
2 2
O C CH CH C O
2 2
O O

Nomenclature of Coordination Compounds: IUPAC Rules
The following rules should be taken in concern when naming the coordination compounds:
1 – The cation is named before the anion when naming a complex,
2 – Lligands are named first alphabetical order and metal atom/ion is named last.
3 – Oxidation state given in Roman numerals follows in parentheses.
4 – Use no spaces in complex name.
5 – The names of anionic ligands end with the suffix –o; -ide suffix changed to –do; -ite suffix changed
to –ito; -ate suffix changed to –ato.
Ligand Name Ligand Name
bromide, Br- bromido -2 carbonato
carbonate, CO
3
chloride, Cl- chlorido -2 oxalato
oxalate, C O
2 4
cyanide, CN- cyanido -2 sulfato
sulfate, SO
4
hydroxide, OH- hydroxido - thiocyanato
thiocyanate, SCN
oxide, O2- oxo -2 thiosulfato
thiosulfate, S O
2 3
fluoride, F- fluorido -2 sulfito
sulfite, SO

3

6 – Neutral ligands are referred to by the usual name for the molecule.
Example: ethylenediamine with the exception; water, H O = aqua; ammonia, NH = ammine; carbon
2 3
monoxide, CO = carbonyl.
5
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid
7 – Greek prefixes are used to indicate the number of each type of ligand when more than one is
present in the complex; di-, 2; tri-, 3; tetra-, 4; penta-, 5; hexa-, 6. If the ligand is a large organic
compound use alternate prefixes: bis-, 2; tris-, 3; tetrakis-,4; pentakis-, 5; hexakis-, 6. The name of the
ligand is placed in parentheses.
8 – If a complex is an anion, its name ends with the –ate appended to name of the metal.
Transition Name if in Name if in
Metal cation complex anion complex
Sc scandium scandate
Ti titanium titanate
V vanadium vanadate
Cr chromium chromate
Mn manganese manganate
Fe iron ferrate
Co cobalt cobaltate
Ni nickel nickelate
Cu copper cuprate
Zn zinc zincate
Examples: [Cr(H O) Cl]SO Pentaaaquachloridochromium(III) sulfate
2 5 4
[Cr(H O) Cl]+2 Pentaaquachloridochromate(III) ion
2 5
K [PtCl ] Potassium tetrachloridoplatinate(II)
2 4
[Co(en) Cl ]Cl Dichloridobis(ethylenediamine)cobalt(III) chloride
2 2
Tetraamminecopper(II) sulfate [Cu(NH ) ]SO
3 4 4
Potassium hexacyanoferrate(II) K [Fe(CN) ]
4 6
Hexaamminechromium(III) hexachloridoiridate(III) [Cr(NH ) ][IrCl ]
3 6 6
Diaquabromidochloridofluoridoiodidochromate(III) ion [Cr(H O) BrClFI] –
2 2
Exercise: Name the following compounds
• [Co(NH ) (H O)F]+2 ; • [Cu(en) ] +2 ; • [Pt(NH )Cl ] – ; • [Cu(en) ][PtCl ]
3 4 2 2 3 3 2 4

6
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid
Isomerism

Isomers: compounds that have the same composition but a different arrangement of atoms. Major
types are ; structural isomers and stereoisomers
1 – Structural Isomers:
a – Coordination-sphere isomers; differ in a ligand bonded to the metal in the complex, as opposed
to being outside the coordination-sphere.
Example: [Co(NH ) Cl]Br vs. [Co(NH ) Br]Cl
3 5 3 5
Consider precipitation
[Co(NH ) Cl]Br(aq) + AgNO (aq)  [Co(NH ) Cl]NO (aq) + AgBr(s)
3 5 3 3 5 3
[Co(NH ) Br]Cl(aq) + AgNO (aq)  [Co(NH ) Br]NO (aq) + AgCl(aq)
3 5 3 3 5 3
b – Linkage isomers; differ in the atom of a ligand bonded to the metal in the complex
Examples: [Co(NH ) (ONO)]+2 vs. [Co(NH ) (NO )]+2
3 5 3 5 2
[Co(NH ) (SCN)]+2 vs. [Co(NH ) (NCS)]+2
3 5 3 5

7
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid
2 – Stereoisomers: Isomers that have the same bonds, but different spatial arrangements
a – Geometric isomers: Differ in the spatial arrangements of the ligands. Have different
chemical/physical properties different colors, melting points, polarities, solubilities, reactivities, etc.

Pt(NH ) Cl
3 2 2
cis isomer trans isomer

[Co(H O) Cl ]+
2 4 2
b – Optical isomers: isomers that are nonsuperimposable mirror images said to be “chiral” (handed)
referred to as enantiomers. A substance is “chiral” if it does not have a “plane of symmetry”

cis-[Co(en) Cl ]+
2 2
Properties of Optical Isomers: Enantiomers; possess many identical properties solubility, melting
point, boiling point, color, chemical reactivity (with nonchiral reagents) different in: interactions with
plane polarized light.

8
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid
Coordination Numbers
The coordination numbers of metal ions in metal complexes can range from 2 to at least 9. In
general, the differences in energy between different arrangements of ligands are greatest for
complexes with low coordination numbers and decrease as the coordination number increases. Usually
only one or two structures are possible for complexes with low coordination numbers, whereas several
different energetically equivalent structures are possible for complexes with high coordination
numbers (n > 6). The following presents the most commonly encountered structures for coordination
numbers 2–9. Many of these structures should be familiar to you from our discussion of the valence-
shell electron-pair repulsion (VSEPR) model because they correspond to the lowest-energy
arrangements of n electron pairs around a central atom. Compounds with low coordination numbers
exhibit the greatest differences in energy between different arrangements of ligands.
Coordination Number 2: Although it is rare for most metals, this coordination number is surprisingly
common for d10 metal ions, especially Cu+, Ag+, Au+, and Hg+2. An example is the [Au(CN) ]− ion, which
2
is used to extract gold from its ores. As expected based on VSEPR considerations, these complexes
have the linear L–M–L structure shown here.
Example: [Ag(NH ) ]+
3 2

Coordination Number 3: Although it is also rare, this coordination number is encountered with d10 metal
ions such as Cu+ and Hg+2. Among the few known examples is the HgI − ion. Three-coordinate
3
complexes almost always have the trigonal planar structure expected from the VSEPR model.

Example: HgI -
3

9
Coordination Chemistry of Transition Metals Dr. Khalil K. Abid
Coordination Number 4: Two common structures are observed for four-coordinate metal complexes:
tetrahedral and square planar. The tetrahedral structure is observed for all four-coordinate complexes
of nontransition metals, such as [BeF ]-2, and d10 ions, such as [ZnCl ]-2. It is also found for four-
4 4
coordinate complexes of the first-row transition metals, especially those with halide ligands (e.g.,
[FeCl ]− and [FeCl ]-2). In contrast, square planar structures are routinely observed for four-coordinate
4 4
complexes of second- and third-row transition metals with d8 electron configurations, such as Rh+ and
Pd+2, and they are also encountered in some complexes of Ni+2 and Cu+2.
Examples: [ZnCl ]-2 [Ni(CN) ]-2
4 4
Coordination Number 5: This coordination number is less common than 4 and 6, but it is still found
frequently in two different structures: trigonal bipyramidal and square pyramidal. Because the energies
of these structures are usually rather similar for most ligands, many five-coordinate complexes have
distorted structures that lie somewhere between the two extremes.
Examples:Fe(CO) [VO(acac) ]-
5 2
Coordination Number 6: This coordination number is by far the most common. The six ligands are
almost always at the vertices of an octahedron or a distorted octahedron. The only other six-coordinate
structure is the trigonal prism, which is very uncommon in simple metal complexes.
Example: [CoF ]-3
6
10