11250
J. Phys. Chem. 1996, 100, 11250-11254
Structural Investigations of trans-Rh(PY3)2(CO)X (X ) F, Cl, NCO; Y ) H, Me, Ph) Using Density Functional Theory and X-ray Analysis Andrzej Wierzbicki,*,† Edward A. Salter,‡ Norris W. Hoffman,† Edwin D. Stevens,§ Liem Van Do,† Margaret S. VanLoock,‡ and Jeffry D. Madura† Department of Chemistry, UniVersity of South Alabama, Mobile, Alabama 36688; Department of Chemistry, Spring Hill College, Mobile, Alabama 36608; and Department of Chemistry, UniVersity of New Orleans, New Orleans, Louisiana 70148 ReceiVed: October 2, 1995; In Final Form: May 2, 1996X
A study of trans-Rh(PY3)2(CO)X (X ) F, Cl; Y ) H, Me) and trans-Rh(PH3)2(CO)NCO using density functional theory is presented together with the results of new high-resolution, low-temperature x-ray data analysis of trans-Rh(PPh3)2(CO)X (X ) F, NCO). In our calculations, we employed the Becke-exchange functional with the Lee, Yang, and Parr correlation functional (B-LYP) and the double-ζ plus polarization basis set (DZP). Our optimized complexes have a slightly distorted square-planar structure with moderate bending of the phosphine ligands toward the uninegative ligand X-. The predicted structures compare favorably with experimental data for trans-Rh(PPh3)2(CO)X (X ) F, Cl, NCO). New X-ray data for trans-Rh(PPh3)2(CO)X (X ) F, NCO) clearly indicate a nonlinear configuration of P-Rh-P atoms, in agreement with our calculations. The apparent linear configuration of P-Rh-P atoms recently reported for the structure of the orthorhombic form of trans-Rh(PPh3)2(CO)Cl is most likely an artifact resulting from the disorder and symmetry imposed by the space group of the crystal structure. Computationally predicted vibrational frequencies ν(C-O) and ν(C-N) (for X ) NCO) compare well with solution-phase FTIR data.
Introduction Previously we have presented a comparative study of some simple model rhodium(I) Vaska complexes using Hartree-Fock (HF) and many-body perturbation theory (MBPT(2)) and various density functional methods (DFT) to analyze the suitability of computational techniques in producing reliable structural and vibrational predictions for these complexes.1 Density functional methods have been found to be both very reliable and computationally cost effective on medium-size to large molecules.2-7 In this study, using the DFT model determined earlier to be suitable for our model system,1 we focus on the complex properties as a function of ligand substitution and the substituent on the phosphorus atom. We present the results of a gas-phase computational study of the model rhodium(I) Vaska complexes shown below, trans-Rh(PY3)2(CO)X (X ) F, Cl, NCO; Y ) H, Me). X Y 3P
Rh
PY3
CO
The structures are distorted 16-electron square-planar complexes formed from a d8 central metal species and four σ-donor ligands. We have computed optimized structures and harmonic vibrational frequencies for the model complexes using density functional theory as implemented in the Gaussian92/DFT8 and Gaussian949 computational chemistry programs. DFT geometries and harmonic vibrational frequencies for many small molecules have been found to compare favorably with experiment and with that of traditional HF and MBPT(2) frequencies. * Author to whom correspondence should be addressed. † University of South Alabama. ‡ Spring Hill College. § University of New Orleans. X Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(95)02918-2 CCC: $12.00
A harmonic vibrational frequency calculation costs roughly the same for DFT methods as for the HF method, and DFT frequencies are often more accurate than the more costly MBPT(2) frequencies, at least for small molecules in the standard 6-31G* basis.10 Previously we have shown that this is a reliable method for describing trends in the structure and vibrational frequencies of such complexes using a DZP quality basis set.1 Computational Details Density functional calculations using Gaussian92/DFT8 and Gaussian949 were carried out on a Cray C90. We employed the Becke-exchange functional11 with the Lee, Yang, and Parr correlation functional12 (B-LYP) and the double-ζ plus polarization basis set13-15 (DZP) in all electron calculations. The default grid option was chosen for numerical integration of matrix elements. Calculations for the PH3 isocyanato complex involved a total of 116 electrons and 172 contracted Gaussian basis functions. Cs symmetry was imposed during geometry optimizations using the Berny algorithm. We computed analytic second derivatives at every point and imposed symmetry to avoid convergence difficulties among rotational conformations of the phosphine groups (estimated barrier to rotations
