Transition Metals. Tuesday 09/22/15. Tuesday, September 22, 15

Transcription

1 Transition Metals Tuesday 09/22/15

2 Agenda Topic Colored Complexes Topic First Row Transition Elements handout (this will be classwork for Wednesday & Thursday)

3 The Periodic Table - The Transition Metals First-row d-block elements Colored complexes Ms. Thompson - HL Chemistry Wooster High School

4 Topic 13.2 Colored complexes The d-sublevel splits into two sets of orbitals of different energy in a complex ion Complexes of d-block elements are colored, as light is absorbed when an electron is excited between the d orbitals The color absorbed is complementary to the colored observed.

5 Nature of science Models and theories - the color of transition metal complexes can be explained through the use of models and theories based on how electrons are distributed in d-orbitals. Transdisciplinary - Color is linked to symmetry can be explored in the sciences, architecture and the arts.

6 Energy of d orbitals In an isolated atom, d orbitals have the same energy but in a complex ion, they split into two sublevels. The electronic transitions between these sublevels leads to an absorption and emission of photons of visible light, which are responsible for the color of the complex.

7 Why wavelengths of visible light are absorbed when passed through a solution with a transition element In [Cu(H2O)6] 2+, the o required to promote an electron to the higher set of d orbitals corresponds to a wavelength of nm.

8 Formation of complex ions Ions of the d-block elements attract species that are rich in electrons (ligands) because of their small size.

9 Formation of complex ions Ligand: a neutral molecule or anion which contains a nonbonding pair of electrons. H 2 O is a common ligand The word ligand is derived from ligandus, the Latin word for bound Most (but not all) transition metal ions exist as hexahydrated complex ions in aqueous solutions (i.e. [Fe(H 2 O) 6 ] 3+ ) Ligands can be replaced by other ligands (such as NH 3 or CN - ).

10 Coordination number: the number of lone pairs bonded to the metal ion. L: M n+ :L Shape: linear Coordination # = 2

11 Coordination number: the number of lone pairs bonded to the metal ion. L: :L M n+ L: :L Shape: square planar Coordination # = 4

12 Coordination number: the number of lone pairs bonded to the metal ion. L.. M n+ Shape: tetrahedral Coordination # = 4 L:.. L :L

13 Coordination number: the number of lone pairs bonded to the metal ion. L: L.. :L L: M n+.. L :L Shape: octahedral Coordination # = 6

14 Coordination number: the number of lone pairs bonded to the metal ion. Examples: state the coordination numbers of the species below. [Fe(CN) 6 ] 3- [CuCl 4 ] 2- [Ag(NH 3 ) 2 ] +

15 Coordination number: the number of lone pairs bonded to the metal ion. Examples: state the coordination numbers of the species below. [Fe(CN) 6 ] 3-6 [CuCl 4 ] 2-4 [Ag(NH 3 ) 2 ] + 2

16 Colored complexes

17 More examples: fill in the ligand, coordination number and oxidation number in the table below. Complex! Ligand! Coordination number! Oxidation number of central ion! Shape (more on this next unit)! Fe(H 2 O) 6 ] 3+!!!! octahedral! [CuCl 4 ] 2-!!!! tetrahedral! Co(NH 3 ) 6 ] 3+!!!! octahedral! [Ag(NH 3 ) 2 ] +!!!! linear! MnO -! 4!!! tetrahedral! Ni(CO) 4!!!! tetrahedral! PtCl 2 (NH 3 ) 2!!!! square planar!

18 More examples: fill in the ligand, coordination number and oxidation number in the table below. Complex! Ligand! Coordination number! Oxidation number of central ion! Shape (more on this next unit)! Fe(H 2 O) 6 ] 3+!!!! octahedral! [CuCl 4 ] 2-!!!! tetrahedral! Co(NH 3 ) 6 ] 3+!!!! octahedral! NH3 [Ag(NH 3 ) 2 ] +!!!! linear! H2O Cl- NH3 O2- CO MnO 4 -!!!! tetrahedral! Ni(CO) 4!!!! tetrahedral! PtCl 2 (NH 3 ) 2! Cl-! & NH3!! square planar!

19 In the free ion the five d-orbitals are degenerate (of equal energy). However, in complexes the d orbitals are split into two distinct energy levels. ΔE Note that this is the splitting pattern for octahedral complexes.

20 The energy difference between the levels corresponds to a specific frequency and wavelength in the visible region of the electromagnetic spectrum. ΔE=hf Note that this is the splitting pattern for octahedral complexes.

21 The observed color is across the color wheel from the absorbed color. Color Wheel

22 The energy separation between the orbitals and hence the color of the complex depends on the following factors: 1) Nuclear charge (based on identity of the central metal ion)

23 2) Charge density of the ligand [Ni(H 2 O) 6 ] 2+ [Ni(en) 3 ] 2+

24 Ex: NH 3 has a higher charge density than H 2 O and so produces a larger split in the d sublevel. [Cu(H 2 O) 6 ] 2+ absorbs red-orange light and appears pale blue [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ absorbs the higher energy yellow light and appears deep blue!i #!<!Br #!<!Cl #!<!F #!<! OH #!<!H 2 O!<!NH 3!<!en!<!NO 2#!<!CN #! Weak-field ligands (small Δ) Strong-field ligands (large Δ)

25 What is en? Colored complexes It is part of a class of ligands called bidentate ligands (more than one atom of the ligand is involved in the bonding to the central atom you don t need to worry about these at this level).

26 3) Number of d electrons present (and hence the oxidation # of the central ion) Mn 2+ Mn 3+ Mn 4+ Mn 6+ Mn 7+

27 4) Shape of the complex ion Electric field created by the ligand s lone pair of electrons depends on the geometry of the complex ion

28 If the d sublevel is completely empty, as in Sc 3+, or completely full, as in Cu + or Zn 2+, no transitions within the d sublevel can take place and the complexes are colorless.

29 NOTE: it is important to distinguish between the words clear and colorless. Neither AP, nor IB, will give credit for use of the word clear (which means translucent) when colorless should have been used. Think about it, something can be pink and clear colorless means something else. Both are clear. Only the beaker on the left is colorless.

30 Practice Problem A certain first row transition metal ion forms many different colored solutions. when four coordination compounds of this metal, each having the same coordination number, are dissolved in water, the colors of the solutions are red, yellow, green, and blue. Further experiments reveal that two of the complex ions are paramagnetic with four unpaired electrons and the other two are diamagnetic. What can be deduced from this information about the four coordination compounds

31 Topic 13.2 Coloured complexes The d-sublevel splits into two sets of orbitals of different energy in a complex ion Complexes of d-block elements are coloured, as light is absorbed when an electron is excited between the d orbitals The colour absorbed is complementary to the coloured observed.