Structure and bonding of Group 13 monocarbonyls

The geometries and vibrational frequencies of the lowest-lying spin-doublet and spin-quartet states of the monocarbonyl and isocarbonyl complexes of the elements of Group 13 have been studied using local density-functional calculations within the linear combination of Gaussian-type orbitals framework. An analogy is drawn between the familiar sigma donation/pi-back donation mechanism used to describe the bonding in transition-metal carbonyls and the bonding in these main-group molecules. Changes in orbital populations and bond orders upon complexation have been used to quantify this idea. The results strongly suggest that the species detected by the observation of characteristic metal-carbonyl stretching frequencies in matrices containing boron, aluminium and gallium together with carbon monoxide are the carbonyl complexes rather than the isocarbonyl isomers. The ground state of BCO is predicted to be a (4) Sigma(+) State but the ground states of the remaining monocarbonyls are likely to be spin doublets. The 2II state of BCO and AlCO may be unstable to bending probably because of the repulsive interaction between the metal s(2) electrons and the donor electron pair on the ligand. This repulsion is much reduced in the (4) Sigma(+) State and together with increased a-back donation this results in considerably stronger M-CO bonds. The sigma-acid and pi-base behaviour of the metals parallels their electronegativity.