Subject Chemistry Paper No and Title Module No and Title Module Tag 11 : Inorganic Chemistry 3 : Structure of metal carbonyls and 18-electron rule applied to them CHE_P11_M3
TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Techniques used for the determination of metal carbonyls 3.1. Structure of mononuclear metal carbonyls 3.2. Structure of dinuclear metal carbonyls 3.3 Structure of trinuclear metal carbonyls 3.4 Structure of tetranuclear metal carbonyls 4. 18-Electron rule 4.1 18-Electron rule applied to metal carbonyls 5. Summary
1. Learning Outcomes After studying this module, you shall be able to Know about structures of various metal carboyl Learn about various techniques used in determining the structure of metal carbonyl Identify structure of metal carbonyl Evaluate electron count of a complex 2. Introduction Carbonyl ligand displays a variety of bonding modes in their metal complexes such as terminal and bridging. Due to large number of bonding modes, the structural determination of metal carbonyl complexes is extremely important. The structures of metal carbonyls are determined with the help of various spectroscopic and diffraction studies such as X-ray diffraction, infrared spectroscopy and electron diffraction studies. Mononuclear metal carbonyls have only linear linkage. The polynuclear metal carbonyls have some other type of linkages also such as: i) M M (metal metal) bond ii) bent M CO M linkage. The terminal and bridging carbonyl groups can be distinguished with the help of IR spectroscopy. The terminal carbonyl group exhibits absorption in the range 1850-2125 cm -1 while the bridging carbonyl group does so in the range 1700-1850 cm -1. The shape of a metal carbonyl depends upon the nature of hybridisation on central metal. Tetrahedral, trigonl bipyramdal and octahedral are the common geometries for metal carbonyls. CO is a strong ligand and it has a tendency to pair up the electrons of the metal atom in the inner orbitals and thus metal carbonyls are inner orbital complexes and most of them follow 18- electron rule with few exceptions.
3. Techniques used for the determination of metal carbonyls: The structures of homoleptic metal carbonyls such as V(CO) 6, Cr(CO) 6, Fe(CO) 5, and Ni(CO) 4 have been established by X- ray diffraction, infrared spectroscopy and electron diffraction studies. The structures of Mo(CO) 6 and W(CO) 6 have been established by X- ray diffraction studies. The mononuclear metal carbonyls have only one type O linkage and it is the linear linkage. The polynuclear metal carbonyls have some other type of linkages also in addition to the aforesaid linear linkage such as i) M M (metal metal bond) ii) bent bridging C carbonyl linkage. M M The M M bond has been established by the measurement of magnetic susceptibility and calculation of magnetic moment. The formation of metal metal bond involves the pairing up of unpaired electrons of metals leading to the diamagnetism. All the metal carbonyls whether mononuclear or polynuclear (except V(C0) 6 ) are diamagnetic,i.e., they have no unaired electrons. The presence of bridging carbonyl group in a polynuclear metal carbonyl has been established by infrared spectroscopy. The bridging carbonyl group exhibits absorption peak in the range 1700-1850 cm -1 due to its stretching mode of vibration. On other hand the terminal carbonyl group exhibits absorption peak in the range 1850-2125 cm -1 in the infrared spectrum due to its stretching mode of vibration. Thus the linear and bridging carbonyl groups of polynuclear metal carbonyls can be distinguished with help of infrared spectroscopy. 3.1 Structure of mononuclear metal carbonyls: The mononuclear metal carbonyls have only M C O linear linkage and their structure depends upon the coordination number of metal, i.e., number of carbonyl groups attached to the metal and the type of hybridization on central metal. The structure of Ni (CO) 4 has been established to be tetrahedral from various studies. The nickel atom of this complex is sp 3 hybridised. [Co(CO) 4 ] - and [Fe(CO) 4 ] 2- are isoelectronic to [Ni(CO) 4 ] and hence they also have tetrahedral structure.
The pentacarbonyls such as Fe(CO) 5, Ru(CO) 5 and Os(CO) 5 have trigonal bipyramidal structure arising from dsp 3 hybridisation on the central metal. [Mn(CO) 5 ] - is isoelectronic to Fe(CO) 5 and it also has trigonal bipyramidal structure. The hexacacarbonyls such as V(CO) 6, Cr(CO) 6, Mo(CO) 6 and W(CO) 6 have octahedral structure arising from d 2 sp 3 hybridisation on the central metal. [V(CO) 6 ] -, [Nb(CO) 6 ] -, [Ta(CO) 6 ] - and [Mn(CO) 6 ] + also have octahedral structure. 3.2 Structure of dinuclear metal carbonyls: a) M 2 (CO) 10 type dinuclear metal carbonyls: The infrared absorption spectral and X-rays diffraction studies on Mn 2 (CO) 10 molecule suggest that in this molecule,each Mn atom is directly attached to other Mn atom by a Mn Mn σ-bond which is formed by the linear overlapping of singly occupied d 2 sp 3 hybrid orbitals and five terminal carbonyl groups through a Mn C O coordinate bond. The presence of Mn Mn bond in Mn 2 (CO) 10 molecule is further supported by its diamagnetic character and short Mn Mn distance(2.79 Å).Thus the coordination number of each manganese atom in Mn 2 (CO) 10 molecule is 6.This molecule contains a linear linkage as shown below: O C Mn Mn C O At the same time, Mn 2 (CO) 10 molecule contains two square planar Mn(CO) 4 moieties in staggered conformation and hence the point group of Mn 2 (CO) 10 molecule is D 4d.
It is obvious that Mn 2 (CO) 10 molecule may be regarded as two distorted octahedral sharing one corner. Tc 2 (CO) 10 and Re 2 (CO) 10 have structures similar to Mn 2 (CO) 10. b) Co 2 (CO) 8 : The infrared spectral studies made on Co 2 (CO) 8 in n-heptane solvent suggest that it occurs in two isomeric forms which are in temperature dependent equilibrium with each other. 2.70 Å 2.52 Å (Non-bridged structure) (Bridged structure) At room temperature, both bridged and non- bridged structures of Co 2 (CO) 8 are present in almost equal proportions. At higher temperatures the non-bridged structure dominates while at lower temperatures the bridged structure dominates. The standard enthalpy of conversion of bridged structure into non-bridged structure is +5.5 kj/mol but it is accompanied by an increase in entropy so that there is very little difference in Gibbs energy of two forms of Co 2 (CO) 8. In the bridged structure of Co 2 (CO) 8, each Co atom is attached to three terminal carbonyl groups, two bridging carbonyl groups and one other Co atom. The Co Co bond is formed by the bent or angular overlapping of singly occupied d 2 sp 3 hybrid orbitals of two Co atoms. The Co Co distance is 2.52 Å and coordination number of Co is 6.
(Bridged structure of Co 2 (CO) 8 ) In the non-bridged structure of Co 2 (CO) 8, each Co atom is attached to four terminal carbonyl groups and on other Co atom. The Co Co bond is formed by the linear overlapping of singly occupied dsp 3 hybrid orbitals of two Co atoms and its length is slightly greater (2.70 Å ) due to the absence of bridging carbonyl groups. The coordination number of Co is 5 with trigonal bipyramidal arrangement around it. (Non-bridged structure of Co 2 (CO) 8 ) The X-ray diffraction studies of the crystals of Co 2 (CO) 8 indicates that the molecule has bridged structure. c) Fe 2 (CO) 9 : The infrared absorption spectral and and X- rays diffraction of the crystals of Fe 2 (CO) 9 suggest that in the molecule, each Fe atom is attached to three terminal carbonyl groups, three bridging carbonyls and one other Fe atom. The Fe Fe bond is formed by the lateral overlapping of singly occupied d xy atomic orbitals of two Fe atoms involving four overlap zones and thus it is a δ- bond.
2.46 Å The coordination number of Fe atom in Fe 2 (CO) 9 is 7 rather than 6. Thus the Fe 2 (CO) 9 molecule consists of two octahedral sharing a triangular face containing three bridging carbonyl groups. Os 2 (CO) 9 is suggested to have Os Os bond but with only one carbonyl group. 3.3 Structure of trinuclear metal carbonyls: Iron, ruthenium and osmium of group 8 of the periodic table form trinuclear metal carbonyls of the type M 3 (CO) 12.The X-rays diffraction and infrared absorption spectral studies made on Fe 3 (CO) 12 suggest that the molecule has following structural features: Two Fe atoms are linked by Fe Fe covalent bond. Each of above two Fe atoms is attached to three terminal carbonyl groups. Each of above two Fe atoms is attached to two bridging carbonyl groups. Each of above two Fe atoms is attached to a third Fe atom by covalent bond. There are three Fe Fe bonds of equal lengths (2.80 Å). The third Fe atom is attached to four terminal carbonyl groups and no bridging carbonyl group.
Ru 3 (CO) 12 and Os 3 (CO) 12 have triangular planar arrangement of three metal atoms which are held together by three M M bonds. Each metal atom is attached to four terminal carbonyl groups and no bridging carbonyl group. 3.4. Structure of tetranuclear metal carbonyls: Cobalt, rhodium and iridium of group 9 of the periodic table form tetranuclear metal carbonyls of the type M 4 (CO) 12. In the molecule of Co 4 (CO) 12, Co atoms are present at the corners of a tetrahedron. Each of the three Co atoms present at the corners of a triangular face is attached to two terminal carbonyl groups, two bridging carbonyl groups and three other Co atoms. Thus the coordination number of Co atom is 7.The remaining fourth Co atom is attached to three terminal
carbonyl groups, three other Co atoms and no bridging carbonyl group.thus the coordination number of this Co atom is 6. Rh 4 (CO) 12 has structure similar to Co 4 (CO) 12.But Ir 4 (CO) 12 has different structure. Four Ir atoms are present at the corners of a regular tetrahedron. Each Ir atom is attached to three terminal carbonyl groups and three other Ir atoms but no bridging carbonyl groups. Its structure is given below: Thus the coordination number of each Ir is 6. 4. 18-Electron rule: Sidwick made an attempt to explain the nature of bonding in transition metal complexes on the basis of electronic concept. According to him, the ligands act as Lewis base and the metal ion acts Lewis acid. Each ligand usually donates one electron pair to the metal ion during the formation of complex compound. The complex compounds in which the central metal atom or ion acquires noble gas electronic configuration are stable. On this basis the effective atomic number rule was proposed as follows:
The sum of the electrons on the central metal atom or ion and the electrons donated from the ligands in a complex compound is called the effective atomic number (EAN) of the metal and for stable complexes it is generally equal to the atomic number of next incoming noble gas. EAN = No. of electrons in the central metal atom or ion + No. of electrons donated by ligands For example, the EAN of Cr in Cr (CO) 6 can be calculated as follows: No of electrons in Cr atom = 24 No. of electrons donated by 6CO=2X6=12 EAN of Cr in Cr (CO) 6 =36 Thus the EAN of Cr in Cr (CO) 6 is 36 which is the atomic no. of next incoming noble gas to Cr,i.e., Kr. It is obvious that Cr (CO) 6 obeys EAN rule and hence it is a stable complex. An alternative and more general statement can be given instead of EAN rule as follows: When the central metal ion or atom of a complex compound acquires noble gas electronic configuration (n-1)d 10 ns 2 np 6 there will be 18 electrons in the valence orbitals (or valence shell) and the electronic configuration will be closed and stable. It is known as the 18-electron rule. The complexes obeying the 18-electron rule are quite stable. 4.1 18-Electron rule applied to metal carbonyls The 18 electron rule is very useful in predicting stabilities and structures of organometallic compounds. There are two conventions for counting electrons in complexes: a) Neutral atom or covalent model b) Oxidation state or ionic model Both the conventions have almost equal number of supporters and both the methods lead to exactly the same net result. The covalent model is probably more foolproof because it does not require the correct assignment of oxidation states, which is sometimes a difficult job in the case of organometallic compounds. Let us illustrate above two methods by counting electrons in HCo(CO) 4. According to the covalent model there is covalent bond between Co and H atoms in HCo(CO) 4 and H atom acts as 1e donor ligand. Accordingly, no. of electrons in valence shell of Co in HCo(CO) 4 =9(Co) + 4x2(CO) + 1X1(H) = 18 According to the ionic model there is ionic bond between (OC) 4 Co + and H - ions and H - ion acts as 2e - donor ligand. Accordingly, no. of electrons in valence shell of Co in HCo(CO) 4 = 8(Co + ) + 4x2(CO) + 1x2(H - ) = 18 The steps are generally followed for counting the electrons present in the valence shell of central metal in a metal carbonyl.
i) The electrons present in the valence shell [i.e., (n-1)d, ns and np orbitals] of the central metal atom are counted and then electrons are added to it or subtracted from it depending upon the nature of charge (negative or positive ) present on it in the metal carbonyl. ii) Each terminal carbonyl group contributes 2e to the valence shell of the central metal. iii) The bridging carbonyl group (M CO M) contributes 1e to the valence shell of each metal atom attached by it. iv)the metal-metal (M M) bond contributes 1e to the valence shell of each metal atom. v) According to the covalent model of electron count, H atom, halogen (X) atom and alkyl groups(r) contribute 1e each to the valence shell of central metal. vi)the electrons donated by the conventional ligands to the valence shell of central metal are also counted. vii) The electrons counted in steps (i) to (vi) are added to find out the total number of electrons in the valence shell of the central metal of metal carbonyl. Some examples of electron counts in the metal carbonyls are given in the following table: Metal carbonyl Electron count of central metal atom(covalent Total electron model) Ni(CO) 4 10(Ni)+4x2(CO) 18 Fe(CO) 5 8(Fe)+5x2(CO) 18 Cr(CO) 6 6(Cr)+6x2(CO) 18 Mn 2 (CO) 10 7(Mn)+5x2(CO)+1X1(Mn Mn) 18 Co 2 (CO) 8 9(Co) +3x2(CO)+2X1(bridging CO)+1X1(Co C o) 18 Fe 2 (CO) 9 8(Fe) +3x2(CO)+3X1(bridging CO)+1X1(Fe Fe) 18 Mn(CO) 5 (CH 3 ) 7(Mn)+5x2(CO)+1X1(CH 3 ) 18 Mn(CO) 5 Cl 7(Mn)+5x2(CO)+1X1(Cl) 18 Os 2 (CO) 9 8(Os)+4x2(CO)+1x1(bridging CO)+1X1(Os Os) 18 Os 3 (CO) 12 8(Os)+4x2(CO)+2X1(Os Os) 18 Ir 4 (CO) 12 9(Ir)+3x2(CO)+3X1(Ir Ir) 18 Na 2 [ Fe(CO) 4 ] 10(Fe 2- )+4x2(CO) 18 V(CO) 6 5(V) +6x2(CO) 17 5. Summary The shape of a metal carbonyl depends upon the nature of hybridisation on central metal CO is a strong ligand and it can be bonded to the metal atom in two different ways forming metal carbonyls CO molecule can be coordinated to the metal atom in a linear fashion as M C O Such a carbonyl group is called terminal carbonyl group.
CO molecule can be coordinated to two metal atoms O C simultaneously as. M M Such a carbonyl group is called bridging carbonyl group The terminal and bridging carbonyl groups can be distinguished with the help of IR spectroscopy The terminal carbonyl group exhibits absorption in the range 1850-2125 cm -1 while the bridging carbonyl group does so in the range 1700-1850 cm -1 Tetrahedral, trigonal bipyramidal and octahedral are the common geometries for metal carbonyls The number of electrons in the central metal of a complex compound including those donated by ligands is called its effective atomic number The effective atomic number of the central metal in a metal carbonyl is invariably equal to the atomic of next noble gas to it. When the central metal ion or atom of a complex compound acquires noble gas electronic configuration (n-1)d 10 ns 2 np 6 there will be 18 electrons in the valence orbitals (or valence shell) and the electronic configuration will be closed and stable. It is known as 18- electron rule. Most of the metal carbonyls obey 18-electron rule and hence they are significantly stable. V(CO) 6 is the only metal carbonyl which does not obey 18-electron rule.