An Alternative Approach to the Teaching of Systematic Transition Metal Chemistry
Brian Hathaway Un~vers~ty College Cork, lrela~d
Since the Renaissance in inoreanic chemistrv' of the 1950's. " transition metal chemistry is taught increasingly in terms of the "general principles which characterize the transition metals." 2 . W i t h the rapid developments in organometallic chemistry, which is taught usually within the inorganic chemistry timetable, the teaching of systematic transition metal chemistrv has been relegated frequently to a minor role in the inorganic chemistry teaching course. consequently, it rests with the lecturer in inorganic chemistry to devise ways of redressing this imbalance by any practical means that are available. This article describes one method for correcting this imbalance. The descriptive chemistry of any transition metal element is an account of the metal, its electron configuration and variable valence, its compounds and complexes, how these are oreoared (and analyzed). an account of their structures and boiding, and how they react. Thus: (A) The most common naturally occurring mineral for each transition metal should he given. The most general method of preparing the pure metal should he described, its most important uses mentioned, and the reactions of the metal summarized in the form of a diagram, Figure 1. (B) The electronic configuration of the metaland its variable valence may he summarized as shown4 in Tahle 1. In Tvoe .. A variable valence.. the ..eroun. oxidation state is the maat stal)li~,lower oxidation states exist l)ut art! g d reducing agents lor they are oxidized readily t ~ w kto the group oxidation state; rhe lower th~!oxidimtm state the stronger the reducing prupt.rties. In Type B variable valence, the group oxidation starc is stable hut has oxidizing properliri since one or more lower oxidation states are the most stable. In Type C variable valence, the group oxidation state is never attained, one or more of the lower oxidation states are now the most stable oxidation state. For each transition metal the variable valence is illustrated best by using the relevant volumetric chemistry reactions in which it is involved (i.e. CrV1 +3eCr"') and by the preparative methods that are used to prepare
Electronic Configuration [Arl
-
At,
Inert film -Chromium-
\ t /
Chromium plating Chromium alloys
Figure 1. The reactions of chromium metal
Table 1. Varlable Valence of the First-Row Transition Metals Ti V Cr Mn Fe Ca
Sc
Elements
the compounds of the metal in each oxidation state (see later). (C) A possible list of simple compounds and complexes for a typical first-row transition metal, chromium, is suggested in Tahle 2. The oxide. fluoride. or chloride mav.he eiven as examples of simple compounds; the hexahydrate as an examole of the octahedral soin-free comolex: . . and the hexacyanide as an example of a spin-paired complex and the tetraethvlammonium tetrachloro metal complex as an example of a tetrahedral complex. The descriptiv~chemistryof ali of these compounds will not all be given in detail, hut one or two examples should be included. The final choice of compounds not only allows the maximum opportunity for the lecturer to add hiruwn interests, but alsoallowi fora description of my "unpredictabli~"features in the chemistry of the transition metals. such as the formation cgf the chromd cation C r O P and C I F ~or PC14Cr11'C14 and the introductik of compounds to illustrate the chemistrv of the less common oxidation state such as the dimeric I C ~ ~ ~ ~ C H ~ C O ~ ) ~ H ~ O I ~ . The chemistry of any individual transition metal compound" or complex then inwlves a description (gf the various f r a t u r ~of inwre~tasset out in the "skelemn" of Tahle3. Not
s2d2
sad'
s2d3
s2d4
s2dS
sQ6
s2d'
TYPE B
TYPE A
Ni
Cu
Zn
sZda
sW
s2d'0
TYPE C
Table 2. Suggested Compounds and Complexes to Illustrate the Predictable and Unpredictable Chemlstry of a First Row Transitlon Metal-Chromium Chromium
VI do
V d'
CrOa: CrFe
CrFr
KzCrO,
NaaCrO
he ~y.>ternatica//y from il kuowledgr of the "physical and chemiral properties which characterize the first-row transition metals." T o cover the descriptive chemistry of the ten first-row transition elements in 10 lectures even to the level2 of "Modern Inorganic Chemistry" by Mackay and Mackay or "Basic Inorganic Chemistry" by Cotton and Wilkinson is difficult, quite apart from the boredom that this approach eenerates. If it is realized that a significant proportion of this systematic transition metal chemistry is pridiciable from the chemistrv of the representative compounds and complexes using the features df interest "skelet&" i t becomes unnecessary to repeat this predictable systematic chemistr.~for each-of t h e elements, as long a s a suitable selection of straightforward compounds and complexes are listed for each metal in its various oxidation states. Ilepending on the time available it is unnrcessary for the chemistw of all ten traniition nietnl ions to be discussed. The present author omits the chemistry of copper from the lecture course as being too unpredictable. It is useful, however, to include one example of a straightforward descriptive type systematic transition metal chemistry lecture to emphasize the advantages of the above approach the student. Equally the students should be encouraged to read the textbook versions of the chemistry of the transition metal ions, approached in the above way, in order that they may make their own choices of "features of interest t w e " comnounds to add to the predictable chemistry suggested in ~ a d l 3e and thus make their own contributions to the chemistry involved. At present the author devotes only the last four lectures in a 23-lecture course to the systematic chemistw of the transition metals. Only three metals are studied in detail, and the choice of metal is varied from vear to vear. T h e first lecture involves a texthiwk-~ypesystematic approach. In the second lecture, the same mrtnl is reexamined to select a short list of simple rompounds and complr~xeiwith predictnth features oi interest type properties follt~wedby the pinpointing of some non-prnlictahle feature.; in the chemistry. In the third lecture the chemistry of a second metal is discussed in limns of the "features of inttrrest" tvue cumu~,undsas set out in Table X. In the final lecture a t k r d metal is discussed. Each student contributes (unrehearsed and unforewamed) any information he or she can on the chemistry either factual or via features of interest type properties or from the previous sections of the lecture course. This process is continued to fill the available Table 3. The Chemistry of a Transition Metal Compound or Complex. Skeleton Features of Interest. (1) The preparation of a compound or complex and ih analysis. (2) Electron configuration of the metal. oxidation states, and d"
configuration.
blackboard space and then the information is organized into various areas of the descriptive chemistry and in practice relates to approximately 60-70% of the content of the descriptive chemistry of the particular metal selected. This accounting is pointed out to the students, and they are then asked to read any descriptive account of the chemistry of this element and to suggest one or two compounds of unpredictable chemistry that they consider of interest to add to the list. Conclusions If systematic transition metal chemistry is taught through the predictable chemistry of its simple compounds andcomplexes, via the transition metal chemistry "skeleton," an adequate amount of descriptive chemistry is covered to give the student a "feeling" for the straightforward descriptive 1)Pre~aration:Cr
+ HCI,.
-
Cr(Hz0)eCls
2) Chromium: (ArJ4s2 3d3 Oxidation State I11 - d3 Octahedral tzg3 egO
3) Octahedral
4)
= 4.0 B.M.;No orbital contribution
5) Electronic Spectra-pale green color
6) Infrared spectrum: HzO 3700, 3600, 1650 cm-' 7) Ionic size: d-block contraction
relatively small due
to d-block contraction t crystal-field effect.
- dn configuration
Isomerism: none due to six equivalent ligands B)
Stability: reasonably thermally stable, very stable with respect
to substitution-Cr(NH3)ez+cannot be prepared by dissolving Cr(HzO)&h in 0.88 NHIOH. Readily oxidized and reduced.
(3) Crystal slructure and sterwchemistry 01 the metal. (4) Magnetic propetlies: spinanly magnetic momant and orbital
contribution. Electronic speclra: Orgei diagram. (6) Other spectrascapictechniques, infrared.Raman and nmr spectra (7) Reactivity: (a)substitution.(b) oxidation. (c)reduction. (8) Physical properties: ionic sire. stability, and isomerism. ( 9 ) important uses: (a) labaratory: (b) commercial. (5)
-
3) Laboratory use: a starting material f o Cr"', ~ CrT' and CrVt compounds, i.e. Cr(OH&Cls + SOCIS CrC13 anhydrous hnmereial use: a mordant in the Dye Industry. lure 2. Features of
interest for Cr(H20)&. Volume 56. Number 6, June 1979 / 391
Cr(H,O).CI, CI,ZH,O dark n e e " ICr(H,O),CIICI,H,O pale green
crci, purpie
SOCI,
Cren,CI,
\
ICr(H,Ol,Cl,l
:::rb/
1
K,Cr(NCS).4H,O
\
Na,Cr(OH),
preparation of other chromium(lll1 Figure 3. The use ot Cr(H20)6C13 in complexes as examples of substitution reactions of Cr(HPlsCls.
to illustrate the variable valence of chromium Figure 4. The use of Cr(H.O).CI. through the preparation of chromium compaunds in different oxidation
states.
Literature Cited
chemistry of the first-row transition metals. Against this background, topping-up with compounds and complexes of "unpredictahle" properties emphasizes the novel aspects of the individual chemistry of these elements, and allows the lecturer to include his individual interests into the descriptive chemistry lectures.
392 / Journal of Chemical Education
(11 k i a t e d with the pioneer workofthe late Prof-or R. S. Nyhoh, F.R.S. (21 (a) Cotton, F, A..end wiikinnon, G.. "Advaned 1norganie Chemistry"and"Banie I". organic Chemintry" Wiley. Interscience. (b) Mseksy, K.M., and McKay R.A, "lntroduction to Modern Inorganic Chcrninhy? lnterted Bwks, London. (31 id Huheey,J. E,"InoqanieChemistry.PrineipiaStrueturcsndRutbi*ity."(blPueell, K.,and Katz, J. C., "lno~snicChemi8t.y" (4) Greenwood. N. N., Univenityof Noftingham. Leeture Notes. 1957. ( 5 ) Bell, C. F.,"Synthpaisand Physicalsfudienof InorgsnieCornpoundo." PergamonPr-, Oxford. 1972.
