Carbonyls terminal, backbonding

Complexes 41 and 42 were characterized by their IR and H-NMR spectra, and 41 also by elemental analysis. Table III contains the pertinent spectral data. Noteworthy are the very low energy terminal carbonyl bands for 41 and 42 at 1864 cm-1 (hexane). The weak 7r-accepting abilities of PR3 (R = Et, Ph) allow the lone CO ligand to 77-backbond to the Ti(II) center to a much greater degree. The -NMR spectrum of 41 exhibited a doublet (/H-p = 1.5 Hz) at 8 4.75 due to the coupling of the cyclo-pentadienyl protons with the 31P nucleus, while complex 42 exhibited a broad cyclopentadienyl singlet at 8 4.67. 

In the carbide rhodium clusters, the Rh—C distances of the terminal carbonyl groups are significantly longer than those of the noncarbide derivatives, whereas the corresponding C—0 distances are shorter (see Table II). This trend indicates a decrease in backbonding to the carbonyl... 

The terminal carbonyls that are oriented approximately opposite the Rh-Rh bond are considered to be exceptionally labile due to the dicationic charge that contracts the Rh d-orbitals and reduces r-backbonding. 

It was proposed [82] that the linearity of the semibridging carbonyls can help maximize the overlap between O and Mo in this case and helps in the donation from 1 jt orbital to the /-orbital. However, population analysis [91] of [CpMo(p-CO)2l2 gives small, negative overlap between O and the other Mo and large, positive overlap between C and the other Mo. Additionally, each linear semibridging CO in [CpMo ( 4-C0)2]2 has ca. 0.09 more electrons than the terminal CO in [CpMo(r-CO)2]2 and each Mo atom has ca. 0.18 fewer electrons. These two pieces of information indicate that the additional contact is ir-backbonding rather than a -donation, a description that offers a better explanation of the linearity of the carbonyls. 

---