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Subject Chemistry
Paper No and Title 11 and Inorganic Chemistry-III
Module No and Title 2 :Routes for the synthesis of metal carbonyls
Module Tag CHE_P11_M2
Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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TABLE OF CONTENTS
1. Learning Outcomes 2. Introduction 3. Direct combination of carbon monoxide and metal 4. Reductive carbonylation of transition metal salts 5. Synthesis using [Fe (CO)6 ] 6. Photolysis of lower metal carbonyls 7. Thermolysis of lower metal carbonyls 8. Abstraction of CO from organic compounds 9. Synthesis of heteronuclear metal carbonyls 10. Synthesis of cationic metal carbonyls 11. Synthesis of anionic metal carbonyls 12. Synthesis of metal carbonyl hydride 13. Synthesis of metal carbonyl halides 14. Summary
Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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1. Learning Outcomes
After studying this module, you shall be able to
• Know different ways for the synthesis of metal carbonyl complexes. • Learn methods like photolysis, thermolysis, reductive carbonylation of metal carbonyls
2. Introduction
[Ni(CO)4] was the first metal carbonyl to be discovered and prepared in the laboratory in the year 1890, which was discovered accidently when Ludwig Mond observed that nickel valves were being eaten up by carbon monoxide gas passing through them. He considered that it happened due to reaction between nickel and carbon monoxide. Then he deliberately heated nickel powder in a current of carbon monoxide gas. Consequently a volatile compound Ni(CO)4 was formed as a water clear liquid which decomposes around 180oC to pure nickel. Metal carbonyls can be prepared by the direct reaction between the finely divided metal with carbon dioxide gas under appropriate conditions of temperature and pressure. Reductive carbonylation of the salts of transition metals in the presence of various reducing agents also leads to the formation of metal carbonyls.
The reducing agents such as CO, H2, metals (Ag, Cu, Mg, Al, Na, etc), LiAlH4 , Grignard’s reagents can be used in reductive carbonylation reactions. The CO groups of [Fe(CO)5] are loosely bound and hence it can be used for the synthesis of other metal carbonyls. The photolysis or thermolysis of lower metal carbonyls leads the formation of higher metal carbonyls. The abstraction of CO by metal from organic compounds such as alcohols, aldehydes and acid chlorides etc. also leads to the formation of metal carbonyls. The dissociation of the M- M bond of polynuclear metal carbonyl by H2 leads to the formation of metal carbonyl hydride. The reduction of metal carbonyls by alkali metals, alkali metal amalgams or sodiumboroydride in tetrahydrofuran or liquid ammonia solvent produces anionic metal carbonyl. The acidification of anionic metal carbonyl produces metal carbonyl hydride. The reaction of metal carbonyl halide with CO and AlCl3 produces cationic metal carbonyl. Heteronuclear metal carbonyl can be produced by the treatment of anionic metal carbonyl with metal carbonyl halide. Now-a-days there are a number of general routes for the synthesis of metal carbonyls as discussed below.
3. Direct combination of carbon monoxide and metal:
This method is mainly used for the synthesis of homoleptic or binary metal carbonyls. Many transition metals in the finely divided state can combine with carbon monoxide gas under suitable conditions of temperature and pressure forming metal carbonyl complexes. However, only [Ni(CO)4] and [Fe(CO)5] are normally synthesized by this method. Finely divided nickel reacts with carbon monoxide gas at room temperature and pressure.
°!, ! !"# o o �� + 4 �� ��(��)! colourless, mp= -17 C, bp = 42 C Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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The reaction between finely divided iron and carbon monoxide gas at an appreciable rate requires elevated temperatures and pressures.
!""°! ,!"" !"# o o �� + 5�� ��(��)! pale yellow, mp= -20 C, bp = 103 C
Co2(CO)8 can also be prepared by the direct reaction of finely divided cobalt metal with carbon monoxide gas. !""℃ ,!"" !"# 2 �� + 8 �� ��!(��)!
But the yield of Co2(CO)8 in this reaction is low.
4. Reductive carbonylation of transition metal salts:
The oxidation state of metal in metal carbonyls is generally zero. This is due to the fact that CO is a π-acceptor ligand .Any metal carbonyl is stable only when M CO π bond of appreciable strength is formed. It is possible only when metal is present in low oxidation state, preferably zero or negative. Therefore metal carbonyls can be prepared by the reduction of salts of transition metals in the presence of carbon monoxide gas. First of all the salt of transition metal is reduced to produce finely divided metal. Then the finely divided metal reacts with carbon monoxide forming metal carbonyl .Various reducing agents can be selected for this purpose. 4.1 Reductive carbonylation by carbon monoxide: When a metal oxide is heated with carbon monoxide gas at high pressure, metal carbonyl is formed.
!"#℃ ,!"# !"# ���! + 9 �� ��(��)! + 4 ��!
!!"! !"##$!" !"# !"#$"%&!'%" ��!�! + 17 �� ��! (��)!" + 7��!
In this reaction, carbon monoxide acts both as reducing agent and as a carbonylating agent.
4.2 Reductive carbonylation by hydrogen:
Hydrogen gas can be used as a reducing agent in reductive carbonylation reaction.
!"#℃ , !"" !"# 2 ����! + 8 �� + �! ��!(��)! + 2 ��! + 2 �! �
4.3 Reductive carbonylation by metals: The metals such as sodium, silver, aluminium, magnesium, copper, etc. can be used as reducing agent in reductive carbonylation reaction.
!"#℃ ,!"# !"# ���! + 5 �� + 3 �� ��(��)! + 3 ���
Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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!"#℃, !"" !"# 2 ��� + 8 �� + 4 �� ��!(��)! + 2 ��!�
!""℃, !"# !"# ���! + 3 �� + 6 �� � �� ! + 3 ����
!"#℃ , !"# !"# 2 ���! + 10 �� + 2 �� ��!(��)!" + 2 ���!
!""℃, !"" !"# 2 �� + 10 �� + 2�� 2 ��(��)! + 2 ��!�
!""℃ , !"" !"# ���! + 5 �� + 2 �� ��(��)! + 2 ���
!"#$%$&',!"#℃ ,!"" !"# 2 ��(����)! + 12 �� + 3 �� 2 �� �� ! + 2 ��(����)! + 3 ��(����)!
!"#$%&' ����! + 6 �� + 5 �� �� �� ! + 5 ����
!""℃ ,!"" !"# 2���! + 8 �� + 4 �� ��!(��)! + 4���
4.4 Reductive carbonylation by LiAlH4 :
Lithium aluminium hydride can be used as a reducing agent during the synthesis of metal carbonyls by the reductive carbonylation of metal halides.
!!"℃ ,!" !"# ����! + 6 �� + �����! ��(��)! + ���� + ����!
4.5 Reductive carbonylation by organometallic compounds The organometallic compounds such as Grignard’s reagents, trialkylaluminium, etc. can also be used as reducing agent during the synthesis of metal carbonyls by the reductive carbonylation of metal halides. !!!"#$%&'"!()*% !"#$%&' ����! + 6 �� ��(��)!
!"#$%!!"#"$%&'&$% 2 ��(����)! + 10 �� ��!(��)!"
4.6 Reductive carbonylation by sodium benzophenone/ketyl: For the synthesis of manganese carbonyl, sodium benzophenone/ketyl can be used as a reducing agent.
!"#$%& �� + (�!�!)!�� (�!�!)!����
!"#$%& 2 (�!�!)!���� + ����! ��[�� �!�! )! + 2 ����
!""℃ ,!""!"#,!"# 2 ��[�� �!�! )! + 10 �� ��!(��)!" + (�!�!)!�� Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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5. Synthesis of [Fe(CO)5]:
The carbonyl (CO) groups of [Fe(CO)5] are loosely bound and hence labile. They can be replaced by chloride ions on treatment with MoCl6 or WCl6 leading to the formation of
Mo(CO)6 and W(CO)6 respectively.
!""℃ , !"!!" ���! + 3[ ��(��)!] �(��)! + 3 ����! + 9 ��
!""℃ , !"!!" ����! + 3[ ��(��)!] ��(��)! + 3 ����! + 9 ��
6. Photolysis of lower metal carbonlys :
Higher metal carbonyls can be prepared by the photolysis of lower metal carbonyls. For example, when the cooled solution of Fe(CO)5 or Os(CO)5 in glacial acetic acid is irradiated by ultraviolet light, Fe2(CO)9 and Os2(CO)9 respectively are obtained.
!"!!"#!! 2��(��)! ��! �� ! + ��
!"!!"#!! 2 ��(��)! ��! �� ! + ��
These reactions proceed through following steps:
!"!!"#!! �(��)! �(��)! +��
�(��)! + �(��)! �! �� ! (Where M = Fe or Os)
7. Thermolysis of lower metal carbonyls:
A lower metal carbonyl can be transformed into a higher metal carbonyl on heating up to an appropriate temperature.
!"℃ 2 ��!(��)! ��!(��)!" + 4��
!"℃ 3 ��! �� ! 2��! �� !" + 3 �� When the solution of Fe! CO ! is heated up to 70℃ and then cooled, green crystals of ��! �� !" separate out.
!) !"℃ !!)!""#$% 3 ��! �� ! ��! �� !" + 3 ��(��)!
Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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8. Abstraction of CO from organic compounds:
This route of synthesis of metal carbonyls involves abstraction of CO from organic compounds such as aldehydes, alcohols, acid chloride etc. The example given below proceeds through three steps.
!"#!"#$%& !""#$#%& [�ℎ�� ��ℎ!)! + ��!���� [���ℎ��(��ℎ!)! ��!�� ]
!"#!$ !"#$%&'$( !"#$%&!'" [���ℎ��(��ℎ!)! ��!�� ] [���ℎ�� �� ��! ��ℎ!)! + ��ℎ!
!"#$%&'(" !"#$#%&'#(% [���ℎ�� �� ��! ��ℎ!)! [�ℎ�� �� ��ℎ!)! + ��!��
The second step of the reaction is the reverse of migratory insertion. The success of the reaction depends upon the thermodynamic stability of the final metal carbonyl formed and it is greater for a low-valent metal. In the case of acid chloride, the first step is the oxidative addition of C-Cl bond which takes place with great facility. In the case of aldehydes, the first step is the oxidative addition of aldehyde C-H bond which takes place with difficulty. It is much difficult for a metal to break C-C bond and, therefore, the ketones , R2CO, are reluctant to take part in this reaction.
9. Synthesis of heteronuclear metal carbonyls:
The heteronuclear metal carbonyls can be synthesized by following methods.
9.1 By reaction of anionic metal carbonyl with metal carbonyl halide: The reaction of an anionic metal carbonyl with a metal carbonyl halide of different metals produces a heteronuclear metal carbonyl. !"#$%& ��[�� ��)! + [�� ��)!�� [(��)!���� ��)! + ����
!"#$%& ��[�� ��)! + [�� ��)!�� [(��)!���� ��)! + ���� These reactions are generally carried in tetrahydrofuran solvent in which sodium carbonylates are soluble. 9.2 By heating cationic-anionic metal carbonyls: When a cationic-anionic metal carbonyl containing different metals is heated up to suitable temperature, a heteronuclear metal carbonyl is formed with loss of carbon monoxide. 600C + - [Re(CO)6] [Co(CO)4] [(OC)5ReCo(CO)4] + CO
9.3 By heating a mixture of two or more metal carbonyls of different metals: When a mixture of two or more metal carbonyls is heated u to a suitable temperature, a heteronuclear metal carbonyl is formed with loss of carbon monoxide.
!""℃ 3 ��(��)! + ��!(��)!" ��!��(��)!" + ����!(��)!" + 3 �� Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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9.4 By irradiation of a mixture of two metal carbonyls of different metals: When a mixture of [Mn2(CO)10 ] and [Re2(CO)10 ] dissolved in n-hexane solvent is exposed to light, a heteronuclear metal carbonyl is formed.
!!!!"#$! ,!"#!! ��!(��)!" + ��!(��)!" 2 (��)!����(��)!
10. Synthesis of cationic metal carbonyls:
A cationic metal carbonyl can be prepared by the reaction of metal carbonyl halide with CO in the presence of a Lewis acid such as AlCl3 or BF3 .
+ - Mn(CO)5Cl + CO + AlCl3 [Mn(CO)6] [AlCl4]
Cationic metal carbonyls can also be prepared by the protonation of metal carbonyls in strong acids in the presence of Lewis acid such as BCl3.
+ - Fe(CO)5 + HCl + BCl3 [FeH(CO)5] [BCl4]
NMR spectroscopy indicates that proton becomes bonded to the metal atom in the above reaction.
11. Synthesis of anionic metal complexes:
There are four important methods for the preparation of anionic metal carbonyls ,i.e., metal carbonylate complexes. 11.1 By the action of hydroxide ion on metal carbonyls: !"#$ ��(��)! ��![��(��)!] This reaction involves the nucleophilic attack by hydroxide in on the carbon atom of the coordinated CO molecule followed by the transfer of electron to metal atom and formation of carbon dioxide.
Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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11.2 By reduction of metal carbonyls: The reduction of metal carbonyls with alkali metals, alkali metal amalgams or sodium borohydride in tetrahydrofuran or liquid ammonia solvent leads to the formation of anionic metal carbonyl. A few examples are given below:
!"!!"/!"# ��!(��)! ��[��(��)!]
!"!!"/!"# ��!(��)!" ��[��(��)!]
!"#$%& !"#"!!"#$"%/!"# ��!(��)!" ��![��!(��)!"]
!"#$%& !" !"#$"% !""#$%! ��(��)! ��![��(��)!]
!"#$%& !"#"!!"#$"% !" !"# ��(��)! ��[���!(��)!"]
!"#$%& !"#"!!"#$"% !" !"#$"% !""#$%! ��(��)! ��![��!(��)!"]
11.3 By displacement of CO from metal carbonyls: A number of substituted metal carbonyls can be prepared by displacement of CO from a metal carbonyl. !"#$%& ��! �� + �� �� ! ��! � �� �� ! � + �� !"#$%& �� �� ! + ���!�! �� �� �� ! �!�! + ��
11.4 By reductive carbonylation: When NbCl5 or TaCl5 dissolved in diglyme solvent is treated with sodium metal in the presence of carbon monoxide gas at high temperature and pressure, corresponding metal carbonylate complex is formed. !""℃ ,!"" !"# + - NbCl5 + Na +CO [Na(diglyme)3] [Nb(CO)6]
!""℃ ,!"" !"# + - TaCl5 + Na +CO [Na(diglyme)3] [Ta(CO)6] It is noteworthy here that the neutral metal carbonyls of niobium and tantalum are not stable but their carbonyl anions are known.
12. Synthesis of metal carbonyl hydrides:
Metal carbonyl hydrides can be synthesized by following methods. 12.1 Reaction of transition metal carbonyls with hydride ion sources:
!"#$%& !"#"!!"#$"% !" !"# ��(��)!�! [�!�� �� !] 12.2 Direct interaction of metal, hydrogen and carbon monoxide: When finely divided cobalt is treated with a mixture of hydrogen and carbon monoxide gases at 150℃ and 50 atm pressures, cobalt carbonyl hydride is formed.
Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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!"# ℃ ,!" !"# 2�� + 8 �� + �! [���(��)!]
12.3 Cleavage M-M bond by H2: The treatment of Mn2(CO)10 with hydrogen gas at 200℃ and 200 atm pressure produces manganese carbonyl hydride.
!"" ℃ ,!"" !"# ��!(��)!" + �! 2[ ���(��)!]
12.4 Acidification of metal carbonl anions: When a solution containing metal carbonyl anion is acidified, metal carbonyl hydride is formed.
!"!!"/!"# ��!(��)! ��[�� ��)!
!"# ��[�� ��)! [���(��)!]
!"!!"/!"# ��!(��)!" ��[�� ��)!
!"# ��[�� ��)! [���(��)!w Roman , size 14
13. Synthesis of metal carbonyl halides:
The metal carbonyl halides can occur as neutral molecules, anions or anions. Type of metal carbonyl halide Example Neutral metal carbonyl halide [Mo(CO)4Cl2]2 + Cationic metal carbonyl halide [Fe(CO)5Br] - Anionic metal carbonyl halide [Mo(CO)5I]
These three species can be prepared by following typical reactions.
!!!"#$%& [Mo(CO)6] [Mo(CO)4Cl2]2
!"!#$"!!!"#$$%&'($ !"#!#$ + - [Mo(CO)6] [ Et4N] [Mo(CO)5I]
!"#$%&' !" !"!!"#$%& !"# !"# !!!" !"#"$ !"#$!!"#$%& + - Fe(CO)5 [Fe(CO)5Br] [BCl4]
The reaction between a metal carbonyl and a halogen is the most common method for the preparation of metal carbonyl halide.
��(��)! + �! [��(��)!�!]
!"#$%&' ,!"℃ !"#$%&"'( !"!!" ,!"#℃ ��!(��)!" ��(��)!�� [��(��)! ��]!
Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls
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The metal carbonyl halide can also be prepared by the reaction of a metal halide and carbon monoxide gas.
!" !" !"#!!"#$ !" !"℃ !"# !" !"# ����! [��(��)!��!]!
!" !" !"!#$%!!"#"$%!!"#$# !" !"#℃ ����! �� �� ��
5. Summary
• Metal carbonyls can be prepared by the direct reaction between the finely divided metal with carbon dioxide gas under appropriate conditions of temperature and pressure • Reductive carbonylation of the salts of transition metals in the presence of various reducing agents also leads the formation of metal carbonyls • The reducing agents such as CO, H2, metals (Ag, Cu , Mg, Al , Na, etc ), LiAlH4 , Grignard’s reagents can be used in reductive carbonylation reactions • The CO groups of [Fe(CO)5] are loosely bound and hence it can be used for the synthesis of other metal carbonyls • The photolysis or thermolysis of lower metal carbonyls leads the formation of higher metal carbonyls • The abstraction of CO by metal from organic compounds such as alcohols, aldehydes , acid chlorides, etc also leads the formation of metal carbonyls • The metal carbonyls can be transformed into various derivatives such as hydrides, halides • Cationic species, anionic species, on treatment with suitable reagents.
Chemistry PAPER No.11: Inorganic Chemistry-III MODULE No. 2 : Routes for the synthesis of metal carbonyls