STRUCTURE AND BONDING OF GROUP-13 MONOCARBONYLS

The geometries and vibrational frequencies of the lowest-lying spin-do ublet and spin-quartet states of the monocarbonyl and isocarbonyl comp lexes of the elements of Group 13 have been studied using local densit y-functional calculations within the linear combination of Gaussian-ty pe orbitals framework. An analogy is drawn between the familiar sigma donation/pi-back donation mechanism used to describe the bonding in tr ansition-metal carbonyls and the bonding in these main-group molecules . Changes in orbital populations and bond orders upon complexation hav e been used to quantify this idea. The results strongly suggest that t he species detected by the observation of characteristic metal-carbony l stretching frequencies in matrices containing boron, aluminium and g allium 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 monoca rbonyls are likely to be spin doublets. The 2II state of BCO and AlCO may be unstable to bending probably because of the repulsive interacti on 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 s tronger M-CO bonds. The sigma-acid and pi-base behaviour of the metals parallels their electronegativity.