Sintese do cobalto

Disponível somente no TrabalhosFeitos
  • Páginas : 11 (2605 palavras )
  • Download(s) : 0
  • Publicado : 29 de outubro de 2012
Ler documento completo
Amostra do texto
Coordination Complexes of Cobalt
Inorganic Synthesis in the General Chemistry Laboratory
G regwy M. Williams, John Olmsted Ill, and Andrew P. Breksa Ill
~ alifornia tate University, Fullerton, Fullerton, CA 92634
S
T he American Chemical Societv has established puidelines for undergraduate educatio; calling for an increased
emohasis o n inorganic and descriptive chemistry. One possible'wayof implementing t hese changes in our &rricula is to
reorganize the general chemistry sequence. By necessity,
this approach will also require a revision o f t he introductory
laboratory. Typically, laboratory experiments in general
chemistrv emohasize nhvsical and analvtical methods. T o
.------ *~
"
t he student, this frequently translates into a seemingly e udless series of titrationsleadine to extensive and tedious calculations. General chemistr;lahoratories
suffer from the
lack of visually exciting experiments that expose students to
the full experience of chemistry.
These problems can, in part, be diminished by incorporating more chemical synthesis into the introductory lahoratory. Syntheses of coordination complexes are particularly attractive because they are easy tocarry out in high yield, they
involve relatively simple stoichiometries, and they provide a
varietv of hiehlv colored comoounds. Several exoeriments
o n coordinaGon chemistry hake appeared in t his J ournal;
most of these have emphasized the chemistry of Co(II1).
S ehera ( I) a nd Wilson (2)published two of the earliest
experiments. More recently, Alexander and Dorsey (3) a nd
Loehlin et al.(4) described freshman laboratory projects in
which Co(II1) c omdexes a re o repared a nd studied. There
have also b een r eports o n th; synthesis of optically active
Co(II1) complexes ( 5,6), a nd there are brief descriptions of
(
thesynthesesof [Co(NH3)~CI]CIz7). [Co(en)zClz]C1(8),a nd
[Co(en)a]s (9). O ne paper concerns a number of metal acetyla cetonate complexes ( lo), a nd experimentsconcerning Ni
(11,12), F e (13,14), a nd Cu ( 1616) have appeared. Most of
these papers describe the synthesis of just one or two compounds, and emphasize detailed characterization. In this
paper, we describe a n experiment involving synthesis and
spectral studies of a series of [Co(NH&L] complexes ( L =
NH3, Cl-, H 20, NOz-, a nd ONO-) t hat not only gives general chemistry students a nintroduction to inorganic synthesis
hut also allows them to conduct a systematic study on the
effect of different ligands on absorption spectra.
~

~

~

~

-

~

~

d

.

Background
T he preparation of Co(II1) p entamine complexes is carried out most conveniently starting from CoC12 6 Hz0,
which is b oth inexpensive and quite stable. The first step in
the syntheses of thesecomplexes requires oxidation t o convert the metal from the +2 to t he + 3 oxidation state. The
oxidation reaction is best carried out using concentrated
Hz02 i n basic ammoniacal solution. Hydrogen peroxide as an
oxidizing agent generates only OH- as reduced product, and
the presence of ammonia guarantees that the Co3+ will h e
stabilized immediately by formation of amine complex(es).
( TheCo3+ ion is not stable in aqueous media since i t forms
the strongly oxidizing hexaquo complex.)

.

+

Net:

-

H,Oz(aq) + 2e2C02+
2C03+ 12NH3
2C02++ HzOz+ 12NH3

20H2C03++ 2e2[Co(NH3)J3+
2[Co(NH3)J3' 20H-

+

O ne of the six ligands in the octahedral system is s uhstantiallv more labile than the other five. makinn i t oossihle t o
c arry out displacement reactions 6 generatethe various
pentamine complexes:
<

~~~

~

~

~~

-

~

In t his equation, q d enotes t h e charge on the ligand L. I n the
preparations described here, q = 0 (HzO, N H d or q = -1
(ONO-, NOz-, c1-).
While Co(II1) . entamine comolexes a re e enerallr r ather
. .o
from
soluble in water, their chloride s aits can be
solution b y excess HC1.

+

~ [ C O ( N H , ) , L ] '...
tracking img