Theoretical Study of the Electronic Structure of Group 6 [M(CO)₅X]^- Species (X = NH₂, OH, Halide, H, CH₃) and a Reinvestigation of the Role of π-Donation in CO Lability

Abstract

Density functional calculations have been employed to investigate the electronic structure of [M(CO)(5)X](-) species (M = Cr, Mo, W; X = NH(2), OH, halide, H, CH(3)) and to compute CO ligand dissociation energies. The calculations indicate that CO loss is most facile from the cis position, and CO dissociation energies are computed to increase along the series X = NH(2) < OH < F < Cl < Br < I < CH(3) < H. These results are in agreement with available experimental data. Trends in CO dissociation are related to the ability of X to stabilize the unsaturated 16e [M(CO)(4)X](-) species formed. In addition, pi-destabilization of the ground-state [M(CO)(5)X](-) species is equally significant. Analysis of the electronic structure of the 18e species shows that Xpi-dpi 4e destabilization results in hybridization at the metal center which enhances trans M-CO but reduces cis M-CO pi-back-donation. Strong pi-donation from X also induces sigma-antibonding interactions between the metal and the cis CO ligands. A fragment analysis reveals that these effects are strongest for the "hard" fluoride, hydroxide, and amide ligands.

Untitled

“…Owing to the energy and form mismatch between the AOs of the F-and Ir-atoms, the MÀF bond is largely ionic. This conclusion was not reached by Macgregor and MacQueen in their investigation of CO dissociation from [MX(CO) 5 ] À (M Cr, Mo, or W; X NH 2 , OH, halide, H, or Me), who state that the fluoro ligand is the strongest p-donor, although [WF(CO) 5 ] À shows the smallest d p (M)/p p (X) mixing among the halogen derivatives [27]. 5).…”

“…Thus, the octahedral d 6 complexes [MX 2 L 4 ] are subject to dissociation despite their kinetic inertness. Two recent theoretical (DFT) studies mentioned above similarly conclude that the cis destabilizing effect of p-donating ligands in [MX(CO) 5 ] À (MCr,Mo, or W; XNH 2 , OH, or halide) is caused by the stabilization of the 16-electron intermediate [27] [28], which suggests that the push-pull interaction is enhanced in coordinatively unsaturated species. In mer,trans-[ReX(CO) 3 (PPh 3)2 ] (X F, Cl, Br, or I), the rate of exchange of 13 CO at the position cis to the fluoro ligand is roughly two orders of magnitude greater than in the chloro analogue, which has been explained by the stabilization of the fivecoordinate transition state by the p-donor X [40].…”

Reactivity of Molybdenum and Rhenium Hydroxo‐Carbonyl Complexes toward Organic Electrophiles

“…Unfortunately, in some instances, the very p-donor character of the ligand may induce the creation of a coordination vacancy, as is the case in the cis-labilization of octahedral d 6 carbonyl complexes. [8] The presence of ligands with high steric bulk can be used to prevent the formation of OH-bridged species, but it can also obstruct the access of reagents to the OH functionality.…”

Solid‐State Structure, Quantum Calculations and Spectroscopic Characterization of the Hydrogen‐Bonded Complex [Os(bpy)2(CO)(EtO···H‐DMAP)][PF6]2

“…[54,55] As shown in Table 3, comparing model 2, which lacks the hydrogen bond, to model 1a, which has the hydrogen bond, we observe that the strength of the OsϪO interaction is increased and the strength of the CO bond decreased, as a consequence of the H-bond formation. This is indicated by a shortening of the OsϪO bond, a lengthening of the CO bond, and a red shift of the calculated and experimental ν(CO) frequencies (even if the calculated frequencies show the expected systematic over-estimation error).…”