3.4. Covalent bonding and coordinate (dative covalent) bonding
A subsection of Chemistry, 9701, through 3. Chemical bonding
Listing 10 of 93 questions
Complete the electronic structures of the following species. Co Co2+ In an isolated transition metal atom the five d orbitals have the same energy. When a transition metal ion forms a tetrahedral complex the d orbitals are split into two groups of different energies. Complete an orbital energy diagram to show this, indicating the number of orbitals in each group. increasing energy isolated transition metal ion tetrahedral complex Cobalt(forms a six co-ordinate complex containing three water molecules and three chloride ions. Write the formula of this complex showing the overall charge, if appropriate. Explain, with the aid of diagrams, how many isomers of the complex in exist. Platinum(forms a four co-ordinate complex containing two ammonia molecules and two chloride ions. Write the formula of this complex showing the overall charge, if appropriate. This complex exists as two isomers. Draw the structure of these isomers and give their names. One of the isomers in is an important anticancer drug. State which isomer this is and explain why this isomer is effective. isomer reason Transition metal ions often exist as hexa-aqua complexes in aqueous solution. The reactions which involve ligand exchange are reversible. [Cu(H2O)6]2+ + 4NH3 [Cu(NH3)4(H2O)2]2+ + 4H2O Write an expression for the stability constant, Kstab, for this equilibrium. Give its units. Kstab = units = The numerical value for Kstab for this equilibrium at 298 K is 1.20 × 1013. Explain how this value relates to the relative stabilities of the two complexes.
9701_m16_qp_42
THEORY
2016
Paper 4, Variant 2
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The transition elements are able to form stable complexes with a wide range of molecules and ions. State the meaning of transition element. The dorbitals in an isolated transition metal ion are degenerate. In complexes, the dorbitals occupy two energy levels. Complete the diagram to show the arrangement of dorbital energy levels in octahedral and in tetrahedral complexes. energy octahedral complex isolated transition metal ion degenerate d orbitals tetrahedral complex  Sketch the shape of two dorbitals: ● one dorbital from the lower energy level in an octahedral complex ● one dorbital from the higher energy level in an octahedral complex. Use the axes below. z lower energy level x y z higher energy level x y  Edds4– and edta4– are polydentate ligands that form octahedral complexes with Fe3+. edds4– H N N H –O2C CO2 – CO2 – CO2 – edta4– N N CO2 – CO2 – CO2 – CO2 – The formulae of the complexes are – and – respectively. On the diagram of edds4–, circle each atom that forms a bond to the Fe3+ ion in –.  – is red and – is yellow. Explain why the two complexes have different colours. When edds4–is added to Fe3+, the following reaction occurs. [Fe(H2O)6]3++ edds4––+ 6H2OState the type of reaction that occurs. Write an expression for the stability constant, Kstab, of –. Kstab =  The table shows the values for the stability constants, Kstab, of both complexes. complex Kstab / mol–1 dm3 – 3.98 × 1020 – 1.26 × 1025 Predict which of the – and – complexes is more stable. Explain your answer with reference to the Kstab value for each complex. When an excess of edta4–is added to –, the following equilibrium is established. –+ edta4––+ edds4–Calculate the equilibrium constant, Kc, for this equilibrium, using the Kstab values given in the table in .  Kc = 
9701_m21_qp_42
THEORY
2021
Paper 4, Variant 2
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Ruthenium and osmium are transition metals below iron in Group 8 of the Periodic Table. Two different complex ions, X and Y, can form when anhydrous RuCl 3 reacts with water under certain conditions. X and Y have octahedral geometry. Aqueous samples of X and Y react separately with an excess of AgNO3. Different amounts of AgCl are precipitated: • 1 mole of complex ion X produces 2 moles of AgCl • 1 mole of complex ion Y produces 1 mole of AgCl. Complete Table 4.1 to suggest formulae for X and Y. Table 4.1 X Y formula of complex Both complexes react with an excess of bipyridine, bipy, to form a mixture of two stereoisomers of 3+. N N bipy Bipyridine is a bidentate ligand. Draw three-dimensional diagrams of the two stereoisomers of 3+. Use N N to represent the bipy ligand in your structures. Ru Ru shows another ruthenium complex. N Ru(NH3)5 5+ (H3N)5Ru N This complex contains the neutral ligand pyrazine. pyrazine N N Suggest how pyrazine is able to bond to two separate ruthenium ions. Pyrazine is an aromatic compound. The bonding and structure of pyrazine is similar to that of benzene. Describe and explain the shape of pyrazine. In your answer, include: • the hybridisation of the nitrogen and carbon atoms • how orbital overlap forms π bonds between the atoms in the ring. Predict the number of peaks seen in the carbon–13 NMR spectrum of pyrazine. Explain your answer. The overall charge of the ruthenium complex in is 5+. Deduce the possible oxidation states of the two ruthenium ions in the complex. Osmium tetroxide, OsO4, reacts with alkenes in a similar manner to cold dilute acidified MnO4 –. shows a proposed synthesis of a condensation polymer G. COOH step 1 HOOC COCl G OsO4 C6H12O2 ClOC step 2 step 3 Suggest a reagent for step 1. Draw the structure of exactly one repeat unit of the condensation polymer G. The ester linkage should be shown fully displayed.
9701_m24_qp_42
THEORY
2024
Paper 4, Variant 2
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Elements and compounds which have small molecules usually exist as gases or liquids. Chlorine, Cl 2, is a gas at room temperature whereas bromine, Br 2, is a liquid under the same conditions. Explain these observations. The gases nitrogen, N2, and carbon monoxide, CO, are isoelectronic, that is they have the same number of electrons in their molecules. Suggest why N2 has a lower boiling point than CO. A ‘dot-and-cross’ diagram of a CO molecule is shown below. Only electrons from outer shells are represented. C O In the table below, there are three copies of this structure. On the structures, draw a circle round a pair of electrons that is associated with each of the following. a co-ordinate bond a covalent bond a lone pair C O C O C O Hydrogen cyanide, HCN, is a gas which is also isoelectronic with N2 and with CO. Each molecule contains a strong triple bond with the following bond energies. bond bond energy / kJ mol–1 –CN in HCN NN CO Although each compound contains the same number of electrons and a strong triple bond in its molecule, CO and HCN are both very reactive whereas N2 is not. Suggest a reason for this. HCN reacts with ethanal, CH3CHO. Give the displayed formula of the organic product formed. What type of reaction is this? Draw the mechanism of this reaction. You should show all full and partial charges and represent the movement of electron pairs by curly arrows.
9701_s10_qp_21
THEORY
2010
Paper 2, Variant 1
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Questions Discovered
93