Structure and bonding of transition metal-boryl compounds. Theoretical study of [(PH3)(2)(CO)ClOs-BR2] and [(PH3)(2)(CO)(2)ClOs-BR2] (BR2 = BH2, BF2,B(OH)(2), B(OCH=CHO), Bcat)

Quantum chemical DFT calculations using the B3LYP functionals have been car ried out for the electronically unsaturated 16 VE five-coordinate osmium bo ryl-complexes [(PH3)(2)(CO)ClOs-BR2] and the 18 VE six-coordinate complexes [(PH3)(2)(CO)(2)ClOs-BR2] with BR2 = BH2, BF2, B(OH)(2), B(OHC=CHO), and B eat (cat = catecholate O2C6H4). The bonding situation of the Os-BR2 bond wa s analyzed with the help of the NBO partitioning scheme. The Os-B bond diss ociation energies of the 16 VE complexes are very high, and they do not cha nge very much for the different boryl ligands. The 18 VE complexes have onl y slightly lower bond energies than the 16 VE species. The Os-B bond in bot h classes of compounds is provided by a covalent sigma -bond which is polar ized toward osmium and by strong charge attraction. Os-->B pi -donation is not important for the Os-B binding interactions, except for the Os-BH2 comp lexes. The stability of the boryl complexes [Os]-BR2 comes mainly from B<-- R <pi>-donation. which is clearly higher than the Os-->B ct-donation. The i ntraligand charge distribution of the BR2 group changes little when the Os- B bond is formed, except for BH2. The CO ligand in [(PH3)(2)(CO)(2)ClOs-BR2 ] which is trans to BR2 has a relatively weak bond to the osmium atom.