Magadh University, Bodh-Gaya

Dr. Partha Pratim Das Assistant Professor P.G. Department of Chemistry Magadh University, Bodh-Gaya

E-Content Title: Metal Carbonyls

Stream: M. Sc. (2nd Semester)

Email id: [email protected]

Metal- π Complexes

 Metal Carbonyls:

Bonding in coordination compounds is usually the donation of ligand electron pair to the metal center only. However, for some ligands, not only they have filled atomic orbitals, which are donor orbitals, but also they possess some empty orbitals, which are acceptor orbitals, having appropriate symmetry and energy to accept electron density from a central metal atom or ion. Such interactions are called π- - back bonding or π-back donation. This is generally shown by CO, CN , NO, PR3 and alkene-alkyne etc. ligands.

Metal carbonyls are one of the most vastly studied types of metal-π complexes that can be defined as coordination compounds of transition metals with carbon monoxide ligand. Metal carbonyls are toxic by inhalation, skin contact, or ingestion, in part due to their ability to attach to the iron of hemoglobin to give carboxyhemoglobin, which inhibits the binding of dioxygen. They are very useful in synthetic organic chemistry and in homogeneous catalysis reactions.

Bonding in Metal Carbonyls: To Study the nature of the bonding between the metal center and the carbonyl ligand, one must understand the bonding inside the carbonyl ligand itself first. Two popular approaches to study the bonding are discussed below; 1. Valence bond theory: The carbon and oxygen atoms in CO are sp-hybridized with the following electronic configurations; 2 2 1 1 0 C (ground state) = 1s , 2s , 2px , 2py , 2pz 2 2 1 1 0 C (hybridized state) = 1s , (spx) , (spx) , 2py , 2pz 2 2 1 1 2 O (ground state) = 1s , 2s , 2px , 2py , 2pz 2 2 1 1 2 O (hybridized state) = 1s , (spx) , (spx) , 2py , 2pz

One half-filled spx-hybridized orbital of carbon atom overlap with half-filled spx-hybridized orbital of the oxygen atom to form σ bond, while spx-hybridized lone pairs on both atoms remain non-bonding in nature. Moreover, two π bonds are also formed as a result of the sidewise overlap; one between half-

filled 2py orbitals, and the second one as dative or coordinative interaction of fully filled 2pz orbital of oxygen with empty 2pz orbital of carbon.

Hence according to this model, the bonding within the CO molecule can e described as:

Now, Coming to Metal-Carbonyls, the central metal atom or ion provides the required number of empty hybrid orbitals with proper orientation to accept the electron pair from surrounding ligands. For 2 3 example, in [Cr(CO)6] the chromium atom undergoes a d sp hybridization to generate six empty hybrid orbital of equivalent shapes and the same energy. When one of the carbonyl ligands approaches this metal ion with its internuclear axis, suppose along x-axis, the filled hybrid lone pair of electron on carbon atom overlap with one of the two empty hybrid orbitals orientated oppositely in x-direction. Metal-carbon multiple bonds are explained in terms of various resonating structures which consequently reduces the bond strength of the carbon-oxygen bond. Here, it should also be noted that, though there are two hybrid lone pairs (one on carbon and the other on the oxygen); the bonding of carbonyl group with metal takes place via a donation through carbon end always, due to the higher energy of hybrid lone pair on carbon than on oxygen.

2. Molecular orbital theory: There are total three molecular diagrams for carbonyl ligand which were proposed. All three molecular orbital (MO) diagrams are able to explain the nature of metal- carbonyl π-bonding. Three MO diagrams for CO molecules has been proposed so far. • The first molecular orbital diagram of carbon monoxide assumes that the atomic orbitals of carbon and oxygen interact with each other to create molecular orbitals. The electronic configurations of C and O are 1s2, 2s2, 2p2 and 1s2, 2s2, 2p4 respectively.

First proposed molecular orbital diagram of carbonyl ligand

Outer electrons in carbon and oxygen are four and six, respectively. Total ten electrons are to be filled in the molecular orbitals of the carbon monoxide molecule. Higher energy of corresponding atomic orbitals of carbon is due to its lower electronegativity, which makes the bonding and anti-bonding molecular orbitals to receive different contributions from atomic orbitals of carbon and oxygen. Bonding molecular orbitals will be rich in atomic orbitals of oxygen, while anti-bonding molecular orbitals, that are closer to carbon in energy, would be rich in atomic orbitals of carbon. Bonding molecular orbitals will have more characteristics of atomic orbitals of Oxygen and anti-bonding Molecular orbitals would have more characteristics of carbon. Electronic configuration of CO molecule will be σ2s², σ*2s², σ2pz², π2px², π2py², which gives a bond order three i.e. triple bond between carbon and oxygen. Two key points to be noted here that, + o When one electron removed from CO to form CO , the bond order actually increases in practical, which is actually the opposite of what is expected if the electron is lost from the highest occupied molecular orbital (HOMO) of bonding nature. Its bond order should decrease upon removal

of an electron from π2px² or π2py², which suggests that the HOMO of carbon monoxide should be of anti-bonding nature rather bonding. o Second anomaly also arises from the MO diagram of CO ligand which clearly shows that in order to donate electron density from π-bonding molecular orbital; the carbonyl ligand must approach the metal center with its carbon-oxygen inter-nuclear axis perpendicular to x, y or z-axis assigned to the central atom. This explains how the empty π*2px and π*2py could be used to accept electron density from filled d-orbitals of central metal atom or ion. However, in actual practice, the carbonyl ligand binds to the metal center in linear fashion via carbon end only.

Expected nature of σ and π overlap in metal carbonyls from the first MO of CO

• According to second molecular orbital diagram of carbon monoxide, 2s and 2px atomic orbitals of both carbon and oxygen undergo hybridization before they create molecular orbitals. The carbon and oxygen atoms in carbon monoxide are sp-hybridized with the following electronic configurations; 2 2 1 1 0 C (hybridized state) = 1s , (spx) , (spx) , 2py , 2pz 2 2 1 1 2 O (hybridized state) = 1s , (spx) , (spx) , 2py , 2pz The total number of valence electrons in carbon and oxygen are four and six, respectively. Hence, ten

electrons are to be filled in the molecular orbitals of CO molecule. The half-filled spx hybrid orbitals * of carbon and oxygen interact to form σ and σ molecular orbitals; while the fully-filled spx hybrid lone pair orbitals of carbon and oxygen remain non-bonding. Moreover, doubly degenerate sets of π-

bonding and π-antibonding molecular orbitals are also formed due to the sidewise overlap of 2py orbitals and 2pz orbitals.

Bonding molecular orbitals will be rich in atomic orbitals of oxygen, while anti-bonding molecular orbitals that are closer to carbon in energy would be rich in atomic orbitals of carbon. Bonding molecular orbitals will have more characteristics of atomic orbitals of oxygen, while anti-bonding molecular orbitals would have more characteristics of carbon. The molecular orbital diagram carbon monoxide proposed is the following;

Second Proposed molecular orbital diagram of carbonyl ligand o This MO diagram eliminates the possibility of sigma donation through bonding molecular orbital and perpendicular orientation CO ligand as the HOMO is now non-bonding hybrid lone pair rather π-bonding. o This MO diagram explains why the carbonyl group prefers to bond via carbon end in a linear manner. o This MO diagram also explains how the lowest unoccupied molecular orbital (LUMO) π*2pz and π*2py could be used to accept electron density from filled d-orbitals of central metal atom or ion. o The reduced CO stretching frequency of metal coordinated carbonyl can be attributed to the reduced bond order due to the transfer of d-electron density from metal to π* orbital carbonyl ligand. + o However, the increase in bond order when one electron is removed from CO to form CO is still a mystery because the electron is lost from the highest occupied molecular orbital (HOMO) of nonbonding bonding nature, and the bond order should have remained the same.

Nature of σ and π overlap in metal carbonyls

• Third proposed molecular orbital diagram of carbon monoxide is most widely accepted to rationalize its σ-donor and π-acceptor strength. Total number of valence electrons in carbon and oxygen are four and six, respectively. Thus, ten electrons are to be filled in the molecular orbitals of CO molecule.

Total of four singly degenerate σ- molecular orbitals and two doubly degenerate sets of π- molecular orbitals are formed. One doubly degenerate set of π molecular orbitals will be bonding while the other one will be anti-bonding in nature. The nature of σ molecular orbitals is more complex as three out of four are of bonding character. Initially, the 5σ was thought to be of anti-bonding to justify the higher bond order of CO+. However, the 5σ is slightly bonding in nature because there is some mixing with the p atomic orbitals of the right symmetry. Out of four σ- molecular orbitals, only 6σ possesses the anti-bonding character, while 5σ goes with expected bonding characteristics. The 5σ is essentially non-bonding and almost centered on the oxygen atom. Doubly degenerate sets of π-bonding and π-anti-bonding molecular orbitals are also formed due to the sidewise overlap of 2py orbitals and 2pz orbitals. The π-bonding molecular orbitals set will be rich in atomic orbitals of oxygen while anti-bonding molecular orbitals, that are closer to carbon in energy, will be rich in atomic orbitals of carbon atom. Still, the problem that why the bond order increases, when an electron is removed from CO still persists. Because we are removing the electron from a bonding molecular orbital, its bond order must be decreased. The possible explanation for the shortening of the bond after ionization is that the ionization induces a shift of the electron-polarization in CO ligand. In other words, the ionization occurs as the loss of an electron from a σ-HOMO orbital which is mostly carbon-centered; and since the HOMO-σ orbital is only slightly bonding in nature, the loss of bonding character is quite small and could easily be compensated by the advantage in covalent character; i.e. the formation of a positive partial charge on the carbon atom increases the strength of the covalence of the bond and thus decreases the bond length. This enhanced covalent character can also be visualized in terms of better interaction of two atomic orbitals if their energies are comparable. In carbon monoxide molecule the atomic orbitals of oxygen lie energetically a lot below then the atomic orbitals of carbon; But when the CO is oxidized to CO+, the partial positive charge on carbon shifts the atomic orbitals of carbon down in energy, and thereby makes the energies closer to the related atomic orbitals of oxygen, which leads to a stronger interaction when bonds are made. In case of σ- bonding, CO molecule acts as Lewis bases, as it donates electrons and metal atom or ion acts as lewis base, as it accepts electron. The MO diagram shown above is very useful in explaining the bonding between the metal center and carbonyl ligand. The carbonyl ligand uses its HOMO for sigma donation while simultaneously accepts electron density from filled metal d-orbital to its π* LUMO.

Nature of σ and π overlap in metal carbonyls using third MO diagram of CO

This kind of π-back bonding is called synergistic bonding and the phenomena are called synergic effect.

π- back bonding π- bonding

When π- back bonding increases, π- bonding in CO decreases, as a result, (i) Double bond character between Metal-C bond increases and double bond character in CO decreases. (ii) Bond order of M-C bond increases and bond order of CO molecule decreases. (iii) Bond length of M-C bond decreases and bond length of CO molecule increases. (iv) Bond energy of M-C bond increases and bond energy of CO molecule decreases. (v) Stretching frequency of M-C bond increases and Stretching frequency of CO molecule decreases.

• Classifications of Metal Carbonyls:  Based on the ligands present: (1) Homoleptic Metal Carbonyls: Only CO molecule is present as ligand to the central metal atom or ion. Example; [Fe(Co)5], [Ni(CO)4] etc. (2) Heteroleptic Metal Carbonyls: Other ligands are also present along with CO molecule. Example; [HFe(CO)4] etc.  Based on Number of metal ions: (1) Mononuclear Metal Carbonyls: Only one metal atom or ion present in the complex. Example: [Fe(Co)5], [Ni(CO)4] etc. (2) Polynuclear Metal Carbonyls: These are of two types again; (i) Homopolynuclear Metal Carbonyls: Same central metal atom or ion present in more than one in number. Example: [Mn2(CO)10], [Ir4(CO)12] etc. (ii) Heteropolynuclear Metal Carbonyls: More than one type of metal ions are present in the complex. Example: [RuCo2(CO)11] etc.

o Factors affecting back-bonding: 1. Oxidation state of metal: As oxidation state of metal decreases, availability of electron for back-bonding increases, hence back-bonding increases, π-bonding in CO molecule decreases. 2. Nature of other ligands bound to metal: Here all the ligands other than the CO ligand/molecule of our concern do affect the back-bonding, three types of ligands can be present;

(i) Only σ-donor ligand: e.g. NH3, H2O, Py etc. (ii) σ-donor and weak p-acceptor ligand: PH3, PPh3 etc. - + (iii) σ-donor and good p-acceptor ligand: CO, CN , NO , C2H4 etc. Αs we go from, σ-donor to σ-donor and weak π-acceptor to σ-donor and good π-acceptor ligands π- back-bonding in CO molecule decreases, as availability of metal electron for π-back bonding decreases in this direction.

Mononuclear Metal Carbonyls: Coordination complexes which follow EAN rule are stable. Metal carbonyls follow EAN rule.

No of Valence Electron 3e- 4e- 5e- 6e- 7e- 8e- 9e- 10e- 11e- 12e- Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Tc Ru Rh Pd Ag Cu La Hf Ta W Re Os Ir Pt Au Hg Sc Valence Electron: 3, To attain EAN fulfillment need another 15 electron, Y i.e. more than 7 CO ligands around metal centre; Hence can’t be stable due La to steric effect. Ti Valence Electron: 4, To attain EAN fulfillment need another 14 Zr electron, i.e. 7 CO ligands around metal centre; Hence can’t be Hf stable due to steric effect. V Valence Electron: 5, To attain EAN fulfillment need Nb another 13 electron, i.e. more than 6 CO ligands around Ta metal centre; Hence can’t form stable neutral metal carbonyls due to steric effect, but can form anionic metal - carbonyls, e.g. Na[V(CO)6], where the anion is [V(CO)6]

Cr Valence Electron: 6, To attain EAN fulfillment need Mo another 12 electron, i.e. 6 CO ligands around metal W centre; Hence can easily form octahedral metal Metals carbonyls, e. g. [Cr(CO)6], [Mo(CO)6], [W(CO)6]

Mn Valence Electron: 7, To attain EAN Tc fulfillment need another 11 electron, i.e. Re needs 5.5 CO molecules, hence odd electron containing metals cant form neutral mononuclear metal carbonyls, but can form dimeric/ heteroleptic metal carbonyls with anionic ligands, e.g. [HMn(CO)5], where EAN is fulfilled Fe Valence Electron: 8, To attain EAN Ru fulfillment need another 10 electron, Os i.e. 5 CO ligands around metal centre; Hence can easily form penta- coordinated metal carbonyls, e. g. [Fe(CO)5], [Ru(CO)5], [Os(CO)5] Co Valence Electron: 9, To attain Rh EAN fulfillment need another Ir 9 electron, i.e. 4.5 CO ligands around metal centre; Hence can’t form mononuclear Metal Carbonyls, instead forms dimeric and polymeric Compounds following EAN. CO can act both as monodentate and bridging ligand Valence Electron: 10, To attain EAN fulfillment need Ni CO another 8 electron, i.e. 4 CO ligands around metal Pd OC CO M centre; Hence can easily form tetra-coordinated or Pt M OC CO square planar (in specific) metal carbonyls, e. g. OC CO CO [Ni(CO)4], [Pd(CO)4], [Pt(CO)4] Cu Ag Au Zn Cu Hg

Binuclear Metal Carbonyls: The structures of the binuclear metal carbonyls comprise of two metal centers and involve either metal-metal bonds or bridging CO groups or both. For example,

• [Co2(CO)8] is known to exist in two isomers. The first one has metal-metal bond with zero bridging carbonyls; the second one has two bridging CO ligands along with one metal-metal bond.

• [Fe2(CO)9] has three bridging CO ligands and six terminal CO groups attached.

• Furthermore, [M2(CO)10] (M = Mn, Tc, Re) has one metal-metal bond and four CO ligands attached to each of the metal centers.

Polynuclear metal carbonyls: The structures of the polynuclear metal carbonyls comprises of three or more metal centers and involve all bridging, all terminal, or a mixture of two types of CO groups. EAN rule is followed all throughout for each metal center.

 [Ru3(CO)12] cluster comprises of an equilateral triangle of Ru centers, each of which has two axial and two equatorial CO ligands. [Os3(CO)12] has the same structure.

 [Fe3(CO)12] is different, with two bridging CO ligands.  [M4CO12] (M = Co, Rh) consists of a tetrahedral M4 core. Three carbonyl ligands are bridging ligands and nine are terminal.

 [Ir4(CO)12] has no bridging CO ligands groups. The Rh4 and Ir4 clusters are more thermally robust than that of the Co4 compound, reflecting the usual trend in the strengths of metal- metal bond for second and third-row metals vs those for the first row metals.

2−  [Re4(CO)16] has no bridging carbonyl.

 [Os4(CO)16], [Os4(CO)15] and [Os4(CO)14] are somewhat more complex because of non-rigidity. The tetranuclear [Os4(CO)16] is analogs to the cyclobutane with a puckered structure. The X-ray diffraction analysis of [Os4(CO)14] unveiled an irregular tetrahedral Os4 skeleton with four weakly semi-bridging CO groups and four different Os–Os bond lengths. The experimental structure

of [Os4(CO)15] was determined to have a planar butterfly-like geometry consisting of two triangles sharing an edge.

 Hexanuclear [M6(CO)16] (M = Rh, Co) exists with an octahedral core with alternate faces participating in the bridging; i.e. with four triply bridged and twelve terminal carbonyls.

 Vibrational Spectra of Metal Carbonyls: Vibrational spectroscopy is one of the most important methods used for the characterization of metal carbonyls. This technique provides very useful information not only about the structural prototype of different metal carbonyl compositions but also rationalizes the nature of bonding in them. It is quite a well-known fact that the C–O vibration for free carbonyl group (CO gas) is typically denoted as νCO, and absorbs at 2143 cm−1. This C–O absorption shifts downward in general to cover a very wide range of wave number as the carbonyl ligand gets attached to a metal center. This is obviously due to the fact that the energy of the νCO band for the metal carbonyls directly correlates with the strength of the carbon-oxygen bond, and is inversely correlated with the strength of the π-back-bonding between the metal and the carbon. In other words, the molecular orbital diagram of carbonyl group suggests that the highest occupied molecular orbital (HOMO), used for σ-donation is weakly bonding; but the lowest unoccupied molecular orbital, used for accepting d-electron density from metal center is strongly anti- bonding; therefore, the σ-donation does not affect the CO bond order very much but the acceptance of electron density in π* orbital decreases the bond order and consequently the bond strength in a significant way. This effect reduces the force constant of C–O bond, while the magnitude of force constant for M–C will be increased by this back-bonding. Enhancement of back-bonding shifts the metal-carbon and carbon-oxygen stretching to higher and lower values, respectively. The π-basicity of the metal center (and thus the C–O stretching frequency) depends upon a lot of factors like the nature and magnitude of the charge on metal center, and the π-accepting tendency of ligands attached other than the carbonyl. A negative charge on the metal center, or ligands with greater σ-donation and weaker π-accepter strength, are expected to decrease the CO stretching frequency; while an accumulation of positive charge on metal center, or ligands with weaker σ-donation stronger π-accepter strength are bound to increase the CO stretching frequency. For example, in the isoelectronic series of Ti Fe, the hexa-carbonyls show decreasing π-back-bonding as one increases (makes more positive) the charge on the metal. Hence, π-basic ligands increase π-electron density at ⟶ the metal, and improved back-bonding reduces νCO.

2− 1− 1+ 2+ Compound [Ti(CO)6] [V(CO)6] [Cr(CO)6] [Mn(CO)6] [Fe(CO)6] νCO (cm−1) 1748 1859 2000 2095 2204

2− 1− 1+ 2+ Compound [Hf(CO)6] [Ta(CO)6] [W(CO)6] [Re(CO)6] [Os(CO)6] νCO (cm−1) 1757 1850 1977 2085 2190

Differentiation of Terminal and Bridging Carbonyl Groups: The mode of attachment of the carbonyl group to the metal center can also be determined by observed CO stretching frequencies. Terminal carbonyls absorb at the higher wave number in comparison to the bridging ones, which is obviously due to the fact that the extant of back bonding increases with the number of metal centers. Three main modes of attachment of the carbonyl group with the metal center are:

Terminal µ2-bridging µ3-bridging

The νCO follows the order: Terminal CO > µ2-bridging > µ3-bridging. The IR-range absorption for various types of carbonyls groups is listed below.

CO stretching frequencies in different metal carbonyl complexes Bonding mode of CO νCO (cm−1) Free CO 2143 Free CO+ 2184 Terminal CO 2120 - 1850 Symmetric µ2-CO 1860 - 1750 Symmetric µ3-CO 1730 - 1600

For Example, in rhodium carbonyl complexes:

Compound µ1-CO, νCO (cm−1) µ2-CO, νCO (cm−1) µ3-CO, νCO (cm−1) Rh2(CO)8 2060, 2084 1846, 1862 Rh4(CO)12 2044, 2070, 2074 1886 Rh6(CO)16 2045, 2075 1819

General Methods of Preparation:  Direct reaction: Some of the mononuclear carbonyls can be prepared by the direct reaction of carbon monoxide with metal powder.

 Reduction Method: One of the most widely used methods to synthesize metal carbonyls is the reduction of corresponding metal salts in the presence of carbon monoxide.

 From mononuclear carbonyls: Iron pentacarbonyl is sensitive to light and air and can be used to synthesize [Fe2(CO)9] by direct photolysis.

 From Iron pentacarbonyl: Carbon monoxide ligands in [Fe(CO)5] are labile and therefore can be used to synthesize other metal carbonyls.

 From metathesis reaction: Mixed-metal carbonyls can successfully be prepared via a metathesis reaction.

Important Reactions of Metal Carbonyls: Metal carbonyls are important precursors for the synthesis of a mixed carbonyl or some important organometallic complexes.

 Ligand Displacement Reactions: The displacement or substitution of CO ligands can be induced photochemically or thermally by some other donor ligands. The ligand-domain is quite wide and comprises of cyanide (CN−), phosphines, nitrogen donors, and ethers also. • The displacement reaction in 18-electron complexes usually follow a dissociative pathway [SN1 type], via a 16-electron intermediate complex. The ligand-displacement-rate in 18- electron complexes is catalyzed by catalytic amounts of oxidants through the electron-transfer phenomena.

Slow Ln-M-CO Ln-M + CO - r. d. s. (18 e ) (16 e-)

+ L'

Ln-M-L' (18 e-)

• The displacement in 17-electron complexes proceeds via an associative route with a 19- electron intermediate complex. ' L - CO Slow Ln-M-L' Ln-M-CO + L' Ln-M r. d. s. (17 e-) CO Fast (17 e-) (19 e-)

• It is worthy to note that the replacement by bidentate ligands like o-phenanthroline (o- phen) and o-phenylene-bis (dimethyl arsine) (diars) occurs in the multiple of two for carbonyl groups.

 Formation of Carbonylate Anions: It is quite a well-known fact that many 2− − − − carbonylate anions like [Fe(CO)4] , [Co(CO)4] , [Mn(CO)5] and [V(CO)6] follow the effective atomic number (EAN) rule. These ions are generally synthesized either by the reaction of metal carbonyl with a strong reducing agent or by the reaction with strong bases.

 Formation of Carbonylate Cations: Carbonylate cations are not as common as carbonylate anions and can be synthesized either by the protonation of metal carbonyl in strong acids or, by the reaction with carbon monoxide and some Lewis acids.

 Synthesis of Metal Carbonyl Hydrides: Carbonyl hydrides are generally synthesized either by the acidification of the solutions containing the corresponding carbonylate anion, or by the reactions of metal carbonyls with hydrogen. Some of the metal carbonyl hydrides can be prepared by direct reaction of the metal with CO and H2. For example:

 Synthesis of Metal Carbonyl Halides or Metal Halides: The metal carbonyl complexes are relatively unreactive toward many electrophiles; however, most metal carbonyls do undergo halogenation. For example, [Fe(CO)5] forms ferrous carbonyl halides. In some reactions, metal-metal bonds are also broken by halogens. On the basis of the electron-counting scheme used, this can be considered as oxidation of the metal center.

Some metal carbonyls also get decomposed into metal halides when treated with halogens as:

 Formation of Metal Carbonyl Nitrosyls or Metal Nitrosyls: The metal carbonyl complexes react with nitrosyl ligand to form either metal carbonyl nitrosyls or metal nitrosyl complexes.

 Disproportionation Reactions: Many metal carbonyls show disproportionation reactions when exposed to some other coordinating ligands. For instance, [Fe(CO)5] reacts with amines to produce hexaamino iron(II) tetracarbonyl ferrate(−II); or [Co2(CO)8] reacts with amine to form Hexaammine cobalt(II) bis-tetracarbonyl cobaltate(−I). The driving force for the above two reactions can be understood in terms of the ease of formation of the carbonylate ions and the favorable coordination number for iron (II) involved. The disproportionation in both cases generates a positive metal center and a metal center with a negative oxidation state. The carbonyl ligand, being a soft base, prefers to bind to the softer acids, i.e., metal center with negative charge; on the other hand, the ligands with nitrogen as donor site are hard Lewis bases, and therefore, prefer to bind to the harder acid i.e. metal center with more positive charge on it.