Theoretical and experimental characterization of Cr-L multiple bonds (L = O, N, and C)

A combined experimental and theoretical study on the bond characterization of Cr-L (L = O, N, C) multiple bonds is applied to a series of Cr-complexes : [(CO)(4)(Cl)Cr-(I)(CPh)] 1; [Cr-(V)(N)(bpb)] 2, and [Cr-(V)(N-t-Bu)(bpb)( Cl)] 3, where [bpb(2-) = (1,2-bis(pyridine-2-carboxamido)benzene)]; and [(T PPOMe)Cr(O-IV)] 4, where [TPPOMe = (5,10,15,20-p-methoxyphenyl)porphyrin)]. Compounds 1 and 2 were investigated by accurate single-crystal X-ray diffr action. Detailed descriptions of Cr-C-carbyne, Cr-N-nitrido, CT-N-imido, an d Cr-O-oxo bonds will be given based on the natural bonding orbital (NBO) a nalyses and Fermi hole function. The bonding feature of all these multiple bonds is essentially a triple bond character consisting of one sigma and tw o pi bonds. The a character of the Gr-L multiple bonds is a highly polarize d one with the electron density strongly polarized toward L (O, N, C) ligan ds. The pi character of the Cr-L multiple bonds depends on the nature of L ligand, i.e., with electron density polarized toward the Cr and O center fo r Cr-C-carbyne and Cr-O-oxo bond, respectively, whereas electron density is roughly equally distributed at both Cr and N for the Cr-Nnitrido and Cr-N- imido bonds. Bond characterizations are also shown in terms of Laplacian of electron density where the inner valence shell charge concentration (i-VSC C) is embedded. The isovalue surface of zero Laplacian of electron density reveals the shape of such i-VSCC at each chromium atom. Shapes around the C r atom are a pressed disklike for 1 and 3 and inverted square pyramid for 2 and 4. The topological properties associated with the bond critical point (BCP) of Cr-L multiple bonds in these compounds indicate a strong covalent bond character. The order of the binding interaction is Cr-N-nitrido > CT-O -oxo > CT-N-imido > CT-C-carbyne The combined study of experiment and theor y on 1 and 2 demonstrates good agreement between experiment and theory.