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.