Metal carbonyls back-donation

CO is an excellent probe molecule for probing the electronic environment of metals atoms either supported or exchanged in zeolites. Hadjiivanov and Vayssilov have published an extensive review of the characteristics and use of CO as a probe molecule for infrared spectroscopy [80]. The oxidation and coordination state of the metal atoms can be determined by the spectral features, stability and other characteristics of the metal-carbonyls that are formed. Depending on the electronic environment of the metal atoms, the vibrational frequency of the C-O bond can shift. When a CO molecule reacts with a metal atom, the metal can back-donate electron density into the anti-bonding pi-orbital. This weakens the C-O bond which results in a shift to lower vibrational frequencies (bathochromic) compared to the unperturbed gas phase CO value (2143 cm ) [62]. These carbonyls form and are stable at room temperature and low CO partial pressures, so low temperature capabilities are not necessary to make these measurements. 

The metal-carbonyl bond donation and back-donation interactions... 

The stability of carbonyls of a metal in the +3 oxidation state is unusual because d7r-p7r back donation is necessary to stabilize the Ir-C bond, generally only one CO group can be bonded to the electron-deficient Ir3+. 

IR spectroscopy of adsorbed carbon monoxide has been used extensively to characterize the diluted, reduced Cr/silica system [48-54,60,76,77]. CO is an excellent probe molecule for Cr(ll) sites because its interaction is normally rather strong. The interaction of CO with a transition metal ion can be separated into electrostatic, covalent a-dative, and 7r-back donation contributions. The first two cause a blue shift of the vco (with respect to that of the molecule in the gas phase, 2143 cm ), while the last causes a red shift [83-89]. From a measurement of the vco of a given Cr(II) carbonyl complex, information is thus obtained on the nature of the Cr(II)- CO bond. 

Generally, the metals that form stable carbonyl complexes are those in the first transition series from V to Ni, in the second row from Mo to Rh, and in the third row from W to Ir. There are several reasons for these being the metals most often found in carbonyl complexes. First, these metals have one or more d orbitals that are not completely filled so they can accept electron pairs from a electron donors. Second, the d orbitals contain some electrons that can be involved in back donation to the k orbitals... 

In this case, the product is the fac isomer, in which all NH3 ligands are trans to the CO molecules. Ammonia does not form ty bonds to metals because it has no orbitals of suitable energy to accept electron density. Thus, the back donation from Cr in Cr(NH3)3(CO)3 goes to only three CO molecules, and the bond order is reduced even more than it is in Cr(CO)s, where back donation occurs equally to six CO molecules. There is, of course, an increase in Cr-C bond order and stretching frequency in Cr(NH3)3(CO)3 compared to Cr(CO)s. Based on the study of many mixed carbonyl complexes, it is possible to compare the ability of various ligands to accept back donation. When this is done, it is found that the ability to accept back donation decreases in the order... 

The chemical behavior of metal carbonyls is influenced by the nature of other ligands present. A decrease in C-O bond order results from an increase in M-C bond order. If other ligands are present that cannot accept electron density, more back donation to CO occurs, so the M-C bond will be stronger and substitution reactions leading to replacement of CO will be retarded. If other ligands are present that are good iy acceptors, less back donation to the CO groups occurs. They will be labilized and substitution will be enhanced. 

The electronic structures of the metal carbonyls have been investigated vigorously over the past few years because the interaction between CO and metals (76, 137) is essential for an understanding of both organometallic chemistry and CO chemisorption phenomena. There is continuing debate concerning the relative importance of o-donation from the carbon lone pair of carbon monoxide and back donation from metal d electrons into vacant rr ligand orbitals. The most frequently cited data in favor of n back donation are... 

Whilst the number of carbonyl ligand-containing complexes known today is vast, only relatively few have been shown to take part in homogeneously catalyzed reactions. Coordination and activation are therefore not synonymous. The bonding involves donation from the carbon lone pair to an empty metal orbital, and back donation from a filled metal d-orbital to an empty antibonding 77 -orbital of the carbonyl ligand (87). 

Such shifts can be rationalized in terms of the model of synergic a and ir bonding of the CO ligand in metal carbonyls (29) as was proposed by Blyholder (30). The formation of a strong surface bond involves considerable back donation from metal d... 

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