THE TRANSITION ELEMENTS 421
acid, blood-red on treating with potassium thiocyanate, gives a
white precipitate with hydrogen sulphide and gives a dark blue
precipitate with potassium ferrocyanide.
(O, Schol.)
- A chromium atom forms a neutral complex with carbon
monoxide molecules and 1,10-phenanthroline molecules. The
structure of the complex is:
(a) Suggest the shape of the complex.
(b) What feature of the structure of a nitrogen atom makes it
possible for it to take part in this sort of complex?
(c) What type of ligand is 1,10-phenanthroline in the complex?
(d) What is the oxidation state of chromium in this complex?
(e) What is the co-ordination number in the complex?
(f) The complex has no stereoisomers; suggest a reason for this.
(g) Comment briefly on whether or not the complexes could be
expected to be water soluble.
(N,A)
- When cobalt(II) chloride was dissolved in water, a pink
solution A was formed. The addition of concentrated hydrochloric
acid to A gave a blue solution B. If solution A was treated with
concentrated ammonia solution a blue-green precipitate was formed;
upon addition of further ammonia solution followed by the passage
of air through the mixture, an orange-red solution C was produced.
(a) Write down the formulae of the species containing cobalt
which is present in each of A, B and C.
(b) How are the ligands arranged spatially around the cobalt in
AandB?
(JMB, A)
