Self-Consistency versus “Best-Fit” Approaches in Understanding the Structure of Metal Nitrosyl Complexes

Abstract

The metal nitrosyl complexes RuX(CO)(NO)L 2 + (L ) P( t Bu) 2 Me, X ) F -, Cl -, BF 4 -, H 2 O, NCH, H -, no ligand, and CO) have been characterized by computing their structures, relative stabilities, and vibrational frequencies through Becke3LYP calculations on a RuX(CO)(NO)-(PH 3 ) 2 + model complex. In the case of X ) F -, Cl -, BF 4 -, and H 2 O, a square pyramidal (SP) geometry with a bent nitrosyl ligand is preferred. In the case of X ) NCH, H -, and CO two geometries exist as local minima: a trigonal bipyramid (TBP) with a linear nitrosyl ligand and a square pyramid with a bent nitrosyl ligand. The computed relative stabilities of such complexes cannot clearly identify the ruthenium coordination geometry. Nevertheless, the correlation between the experimental and theoretical ν NO stretching frequencies is conclusive in identifying Ru(H)(CO)(NO)L 2 and Ru(CO) 2 (NO)L 2 + as TBP structures and Ru(NCMe)-(CO)(NO)L 2 + as SP. Three sets of additional calculations were also carried out on a selected system (X ) H -). The computational level was increased to CCSD(T), the solvent effect was introduced with a PCM approach, and the real phosphine ligands were considered with a QM/MM ONIOM method.

The ONIOM Method and Its Applications

“…Wu et al examined the geometries of DNA oligomers and showed that the geometries of oligomers obtained by ONIOM-PCM were similar to those obtained by ONIOM calculations that included solvent effects by using an 8.0 Å water box . ONIOM-PCM is efficient for calculating energetics and geometries for chemical reactions in which solvent molecules do not participate in the reaction explicitly. ,,, The properties of excited states, such as absorption and emission spectra, are sensitive to the change of charge distribution by surroundings. Thus, the consideration of polarizability of solvent is essential for such calculations (see section ) .…”

“…591 ONIOM-PCM is efficient for calculating energetics and geometries for chemical reactions in which solvent molecules do not participate in the reaction explicit- ly. 204a, 287,330,592 The properties of excited states, such as absorption and emission spectra, are sensitive to the change of charge distribution by surroundings. Thus, the consideration of polarizability of solvent is essential for such calculations (see section 7).…”

“…In addition to providing reliable structures of organometallic compounds, ONIOM calculations can be used to evaluate the relative energies of isomers. Thus, ONIOM studies have shown that steric interactions control the relative stabilities of trigonal bipyramidal and square pyramidal isomers of Ru­(NCH)­(CO)­(NO)­(P t Bu 2 Me) 2 + , cis , trans , cis and all- cis isomers of some six-coordinate d 0 dioxo compounds, and syn and anti isomers of a Re-based olefin metathesis catalyst . In a representative study of Ru­(CO) 2 L 3 complexes (L = PEt 3 or P i Pr 2 Me), Ogasawara et al examined how the relative energies of the trans -CO and cis -CO isomers (Figure ) are affected by the ligand, L .…”

Quantification of Ni–N–O Bond Angles and NO Activation by X-ray Emission Spectroscopy

“…9,12 A further complication is the limited ability to correlate calculated electronic structures and M−N−O bond angles with IR data. 13 As such, electronic structure characterization of M− NO species generally involves significant spectroscopic and computational efforts. angle is especially important in enzymes such as the reduced forms of nitrite reductase (NiR) that exhibit side-on NO bonding.…”

“…Infrared spectroscopy (IR) is generally used to characterize the presence of a metal nitrosyl, and the IR stretch is sensitive to NO activation/NO bond length and the metal–N–O bond angle. − The contributions from activation and bond angle, however, are difficult to deconvolute and not well-quantified. , A further complication is the limited ability to correlate calculated electronic structures and M–N–O bond angles with IR data . As such, electronic structure characterization of M–NO species generally involves significant spectroscopic and computational efforts.…”