A transition metal atom or ion may be directly bonded to a number of atoms or molecules that surround it (ligands),forming a characteristic pattern.This is the essential structural feature of an important class of so-called coordination or complex compounds. If two or more atoms from one individual ligand form bonds to the same central atom then the ligand is said to form a chelate (Greek chele = crab'claw).The glycinate ion,NH2–CH2–COO–,is a bidentate chelate ligand which can form,for instance,tris-glycinato-chromium(III) complexes.The figure shows one possible structure of such a complex. Oxygen and nitrogen are forced to coordinate to adjacent octahedral positions, as the N – C – C – O chain is too short to "embrace" the chromium ion.
(a) How many different configurational isomers of the complex are possible, not counting optical isomers?
(b) Which of these isomers can be further resolved into optical isomers? Another coordination compound of chromium was analyzed and found to have the following mass composition: 19.5 % Cr, 40.0 % Cl, 4.5 % H, and 36.0 % O. A 0.533 g sample of the compound was dissolved in 100 cm3 of water, and 10 cm3 of nitric acid (2mol dm-3) was added. Excess of silver nitrate solution was then added and the precipitate formed was then filtered, washed, dried and weighed. Its mass was found to be 0.287 g. When a 1.06 g sample was gently heated to 100°C i n a stream of dry air,0.144 of water was driven off. The freezing point of a solution prepared from 1.33 g of the compound and 100 cm3 of water, was found to be –0.18°C. (Molar freezing po int depression of water is 1.82 K kg mol-1). Use all the experimental information to solve the following problems:
(c) Derive the empirical formula of the compound.
(d) Deduce formula for the compound showing the ligands of the chromium ion. Give molar ratios to support your result.
(e) Sketch all possible steric arrangements of the ligands about the chromium ion.
