1-4
Following are stages taking place in usual way of stabilizing metals in low oxidation states. Put in correct order.
A. Phosphines are attached with three organic groups
B. Phosphines bond to metal
C. Phosphines pump electrons back to metal atom
D. Phosphines form a back bond to metal
5-7
Below are stages that occur in usual way and the new way of making metal phosphines.
U if it is the usual way
N if it is the new way
Metal Compounds Which Break the Rules
In the quest for more efficient catalysts, a group of British chemists has uncovered a series of metal compounds with structures that are upsetting conventional theories of chemical bonding. Metal compounds that function as catalysts usually do so in an unusually low oxidation state, in which fewer than the usual number of electrons has been lost. Making such states stable is something of an art.
One way of stabilizing metals in low oxidation states is to form a compound, known as a complex, with phosphine-phosphorus compounds with three organic groups attached. Phosphines stabilize a low oxidation state because, as they bond to a metal, they pump electrons back to the metal atom by forming a “back-bond" to the metal.
Noel McAuliffe, Stephan Godfrey and Tony Mackie of the University of Manchester Institute of Science and Technology have discovered a way of making phosphine complexes directly from metal powders using the reagents phosphine-bromine or phosphine-iodine (Angewandte Chemie, International Edition, p. 919; Journal of the Chemical Society. Chemical Communications, 1992, p. 484 and p. 945). When they examined one of their reagents, triphenyl phosphine-bromine, they discovered that it had a structure which defies conventional theories of bonding (Journal of the Chemical Society. Chemical Communications, 1992, p. 356).
Usually, chemists make metal phosphines by reacting metal bromides and iodides with phosphines. But McAuliffe and his colleagues have discovered that a better way is to react the phosphine first with bromine or iodine, then with the metal itself. The first step forms compounds such as triphenyl phosphine-bromine, (QH PBr2, and trimethyl phosphine-iodine, (C3H3)PI2. These were assumed to be ionic compounds, in keeping with conventional theory, an idea supported by their behaviour in solution, which is that of an electrically conducting material, for example, [(C6Hs)3 PBr] and Br.
However, when Robin Pritchard, a crystallographer at UMIST, analysed the colourless crystals of CgH PBr2 with X-rays he discovered that it was the first ever neutral molecule in which phosphorus forms four covalent bonds and bromine forms two bonds. The iodine compound, (C^H^ PI* has the same chemical structure. Such structures defy the known laws of chemistry, which in a neutral molecule require both the bromine atoms or iodine atoms to be directly bonded to the phosphorus atom at the centre.
It is with compounds such as these that McAuliffe has been making unusual compounds. Zinc power reacts with triethyl phosphine-iodine dissolved in ether to give a 100 per cent yield of a 1:1 zme complex, the first ever zinc phosphine whose structure has been determined. Previous attempts to make such a compound exclude moisture and air, while the new reaction goes easily and smoothly.
Similarly, nickel formed a complex, NiljKCHPl with oxidation state HI, and cobalt produces a complex with a unique pattern of five bonds to the central cobalt atom. Not only that, but a reaction between cobalt powder and tributyl phosphine-iodine yielded crystals not of the expected product but of what the authors call a “frozen transition state”. In these crystals the reaction has been stopped at its halfway stage. X-ray analysis showed that they had frozen the phosphine-iodide reagent in the act of attacking the cobalt atom. One phosphine-iodine molecule had surrendered its iodine atoms to cobalt with backup from another, while a third phosphme-iodine molecule attacked the cobalt from the other side.
In a reaction with manganese powder, the product consisted of two complexed manganese atoms linked together through iodine atoms. While such bridged complexes are quite common, this particular one has one of the manganese atoms in oxidation state II and the other is in oxidation state III.
Stage 1
Stage 2
Stage 3
Stage 4
After reacting with bromine or iodine, the phosphine reacts with the metal itself.
Phosphines react with metal bromines and iodines.
Moisture and air are excluded.