Ground-State Electronic Asymmetry in Cp*W(NO)(η¹-isonitrile)₂Complexes

Abstract

The three Cp*W(NO)(η1-isonitrile)2 complexes [Cp* = η5-C5Me5; isonitrile = CNCMe3 (1), CN-2,6-Me2C6H3 (2), CN-n-C4H9 (3)] possess asymmetrical “piano-stool” molecular structures in the solid state in which their two isonitrile ligands exhibit distinctly different degrees of W→isonitrile back-bonding. Thus, an X-ray crystallographic analysis of 1 as a benzene hemisolvate has established that it has one essentially linear isonitrile ligand having a C−N bond length of 1.159(5) Å and a C−N−C bond angle of 171.4(5)° and one very bent isonitrile ligand (indicative of considerable W→CNCMe3 π-back-bonding) having C−N = 1.200(5) Å and C−N−C = 135.1(4)°. Complex 2 crystallizes with two crystallographically independent molecules in the asymmetric unit, and the isonitrile ligands in each of these molecules exhibit metrical parameters similar to those extant in 1. The presence of the inequivalent isonitrile ligands is also manifested in the IR spectra of the three compounds both in the solid state and in solution. For instance, the Nujol mull IR spectrum of 1 contains absorptions at 2038 and 1853 cm−1, attributable to νCN of the linear and bent isonitrile ligands, respectively. An Et2O solution of 1 exhibits these bands at 2051 and 1855 cm−1. However, their 1H and 13C{1H} NMR spectra indicate that on the NMR time scale all three isonitrile complexes are fluxional in solution. Thus, at room temperature the 1H and 13C{1H} NMR spectra of 1 in C6D6 and C6D5CD3 indicate the equivalence of the two isonitrile carbon atoms attached to the tungsten center and the slight inequivalence of the CMe3 components of the two isonitrile ligands, and these spectra remain invariant to −80 °C. The results of DFT calculations on the model complex CpW(NO)(CNMe)2 reproduce the asymmetric distortion as the optimized geometry from various starting points even though it has a relatively shallow potential well. Interestingly, optimization of the related TpW(NO)(CNMe)2 [Tp = [HB(C3N2H3)3]−] results in a symmetric geometry with isonitrile CNC angles of 177.0° and 177.8°. Hence, it is clearly the interaction of the Cp ligand with the orbitals of the W(NO)(CNMe)2 fragment that allows the system to get to lower energy by skewing the isonitrile bending to one side.