Dative 7r-bonding

C. Ab Initio Cl Models of Simple Double, Charged, and Dative 7r-Bonds... 

The nature of bonding in the cationic metal carbonyls has been investigated by both vibrational and electronic spectroscopy, and molecular orbital calculations have been carried out these are consistent with a bonding scheme for carbon monoxide coordinated to a metal, consisting of a dative cr-bond from carbon to metal, augmented by a synergic metal-to-carbonyl dative 7r-bond (7, 57). 

It was suggested that in addition to the overlap of the filled jr-orbital of the olefin with the vacant s-orbital or sp-hybrid orbital of the silver atom (o-bond), there was also overlap of the filled rf-orbitals of silver with the vacant anti-bonding 7r -orbital of the olefin (a dative 7r-bond). This interpretation of silver-olefin bonding will be discussed more fully in Sect. E. 1. 

A chemical method of detecting dative 7r-bonding to Group IV elements is to measure the relative donor properties of compounds in which such bonding is suspected. A guide to the Lewis basicity of such derivatives is provided by the shift... 

The molecules CH20 and BH3CO show a wider variety of electronic sub-structures than the simple hydrids discussed above in particular CH20 contains a (polar) ir-bond and sp2 hybrids and BH3CO has 7r bonds, sp hybrids and a dative bond. It is of some interest therefore to see how the GHOs behave in these situations. Table 2 contains the relevant orbital exponents. The pattern of orbital contraction for GHOs involved in a X—H bonds is repeated and reinforced by the C—0 a bond orbitals. 

Electronegative atoms adjacent to silicon wonld contract the d orbitals and transfer electrons to form a dative bond. This concept is called pjr-drr or (p-d)7r bonding (see Bonding) and has been nsed to explain a number of unique... 

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. 

The cations 18a and 19a are predicted to be distorted, the bridge atom (7) being tilted towards the double bond of 18 or one double bond of 19 and the hydrogen atom tilted away this is exactly what would be expected if the ions are it complexes 38) in which one double bond forms a dative bond to the 7-carbon atom. It is perhaps surprising that the saturated cation (18a) shows a similar deformation presumably I-strain at the 7-position favours a 7r-complex-like structure in which a C-C a bond acts as donor. 

Since in this work the most studied molecular systems involve the 7r-donor and 7r-acceptor subunits, we will consider the relation between a nonbonding donor-acceptor combination, on the one hand, and the dative bond, on the other hand. 

An examination of Table XII shows that in all cases the M-C interaction is a dative bond, i.e., donation of electron charges from the n orbital of olefin to the vacant s orbital of metal and, simultaneously, back-donation of electron charges from the d orbitals of M to the n orbital of olefin (Fig. 12). This can be interpreted in more detail as follows. When the olefin molecule approaches M+, some electronic charge is transferred from the C=C it orbital to the valence s orbital of M+ at the same time, electrons in the filled d orbitals of metal are transferred to the symmetry-matched 7r orbital of olefin. It can be seen from Table XII that upon adsorption, the electron occupancies of the valence s orbitals of Cu and Ag always increase, whereas the total occupancy of their Ad or 5d orbitals always... 

Since the electron density of nickel in bis(7r-crotylnickel iodide) is high, the formation of five-coordinate complexes is favored for compounds with vacant orbitals readily attached by dative bonds (11). The bases might be classed into two types by this characteristic ... 

Figure 33 Partial MO diagram for a trans dinitrosyl compound focusing on the 7r-interactions. Since the two sets of NO 7T orbitals on each ligand interact with the same metal d orbitals, the metal is only required to contribute two electrons to the bonding. Ooupled with the two ON- M dative covalent interactions (not shown), each nitrosly may be classified as an LX ligand.

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