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.
| A | B | ||
|---|---|---|---|
| 1 | After reacting with bromine or iodine, the phosphine reacts with the metal itself. | ||
| 2 | Phosphines react with metal bromines and iodines. | ||
| 3 | Moisture and air are excluded. |
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Question Focus: Synonym match - 'react the phosphine first with bromine or iodine' = 'After reacting with bromine or iodine'.
Why this is correct: The passage says McAuliffe's team found a better way. It is to react the phosphine first with bromine or iodine, then with the metal itself. 'A better way' that the team discovered is the new method. So this stage belongs to the new way.
Passage Reference: 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.
Common Mistake: Some students choose the usual way, because bromine and iodine also appear in the sentence about the usual method. But in the usual method they are part of the metal compounds. In the new method the phosphine meets them first. This is a Partial truth trap.
Paraphrasing: Passage: 'react the phosphine first with bromine or iodine, then with the metal itself' = Statement: 'After reacting with bromine or iodine, the phosphine reacts with the metal itself'
Tips: For each stage, find the part of the text that describes it. Then check which method that part of the text is about.
Key Vocabulary:
- react = change chemically when mixed with something
- colleague = a person you work with
- itself = the pure metal, not a compound of it
Question Focus: Signal word - 'Usually' tells you this sentence describes the standard method.
Passage Reference: Usually, chemists make metal phosphines by reacting metal bromides and iodides with phosphines.
Why this is correct: The passage says Usually, chemists make metal phosphines by reacting metal bromides and iodides with phosphines. The word 'Usually' shows this is the normal, older method. So this stage belongs to the usual way.
Common Mistake: Some students choose the new way, because the new method comes right after this sentence and uses bromine too. But the sentence about metal bromides starts with 'Usually'. This is a Context shift trap.
Paraphrasing: Passage: 'Usually, chemists make metal phosphines by reacting metal bromides and iodides with phosphines' = Statement: 'Phosphines react with metal bromines and iodines'
Tips: Find the same step in the text. Then look for a word that shows whether the writer means the old method or the new one.
Key Vocabulary:
- usually = most of the time, as normal
- bromide = a compound made with bromine
- iodide = a compound made with iodine
Question Focus: Contrast - 'Previous attempts' needed to keep out moisture and air, 'while the new reaction goes easily'.
Passage Reference: Previous attempts to make such a compound exclude moisture and air, while the new reaction goes easily and smoothly.
Why this is correct: The passage says Previous attempts to make such a compound exclude moisture and air. Previous attempts used the older method. The same sentence says the new reaction goes easily and smoothly. So keeping out moisture and air belongs to the usual way.
Common Mistake: Some students choose the new way, because this sentence is in the paragraph about McAuliffe's new compounds. But the sentence contrasts the two, and only the earlier attempts needed this step. This is a Context shift trap.
Paraphrasing: Passage: 'Previous attempts ... exclude moisture and air' = Statement: 'Moisture and air are excluded'
Tips: Find the words about moisture and air in the text. Then check if they describe the older method or the new one.
Key Vocabulary:
- exclude = keep out
- moisture = small amounts of water in the air
- previous = earlier
General Tips:
- There are only two groups here, so each statement belongs to one method or the other.
- One group can be used more than once.
- Look for words that show whether a method is old or new, and check which method each sentence describes.
- Both methods use similar chemicals. Check the order of the steps, not just the chemical names.
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