Computational Analysis of n→π* Back-Bonding in Metallylene–Isocyanide Complexes R₂MCNR′ (M = Si, Ge, Sn; R =tBu, Ph; R′ = Me,tBu, Ph)

Abstract

A detailed computational investigation of orbital interactions in metal-carbon bonds of metallyleneisocyanide adducts of the type R2MCNR´ (M = Si, Ge, Sn; R, R´ = alkyl, aryl) was performed using density functional theory and different theoretical methods based on energy decomposition analysis.Similar analyses have not been carried out before for metal complexes of isocyanides even though they have for long been of common practice when investigating the metal-carbon bonds in related carbonyl complexes. The results of our work reveal that the relative importance of -type backbonding interactions in these systems increases in the sequence Sn < Ge << Si and, in contrast to some earlier assumptions, the -component cannot be neglected for any of the systems investigated.While the fundamental ligand properties of isocyanides are very similar to those of carbonyl, there are significant variations in the magnitude of different effects observed. Most notably, when coordinated to metals, both ligands can display positive or negative shifts in their characteristic stretching frequency. However, because isocyanides are stronger -donors, the metal-induced changes in the CN bonding framework are greater than that observed for carbonyl. Consequently, isocyanides readily exhibit positive CN stretching frequency shifts even in complexes where they function as -acceptors, and the sign of these shifts is alone a poor indicator of the nature of the metal-carbon interaction. On the other hand, the relative -character of the metal-carbon bond in metallylene-isocyanide adducts, as judged by the natural orbitals of chemical valence as well as by partitioning the orbital interaction energy, was shown to have linear correlation with the shift in CN stretching frequency upon complex formation. The details of this correlation show that -back-[2] donation contributions need to exceed 100 kJ mol 1 in order for the shift in the CN stretching frequency of metallylene-isocyanide adducts to be negative.

(NHC)Si═C═N–R: A Two-Coordinated Si0-Isocyanide Compound as Si(NHC) Transfer Reagent

“…Recently, also the 1-silaketeniminyl lithium compounds R 2 B­(Li)­SiCNR′ (BR 2 = diaminoboryl; R′ = 1-adamantyl, 2,6-diisopropylphenyl) were reported, and reductive coupling of two isocyanides was achieved at a bis­(NHSi) scaffold (NHSi = N-heterocyclic silylene) . Structural, spectroscopic, and quantum chemical studies indicate that the bonding in R 2 Si­(CNR) can be described by an allenic and a zwitterionic resonance structure (Figure a), providing a rationale for the observed isocyanide dissociation and silylene-type reactivity of these compounds, ,, which differs from the addition chemistry of the SiC bonds of silenes and 1-silaallenes . These studies implied that electropositive substituents on silicon, which are known to decrease the singlet–triplet gap of silylenes, should increase the contribution of the allenic form, leading to a stronger Si–C isocyanide bond.…”

Cationic Two-Coordinate Complexes of Pd(I) and Pt(I) Have Longer Metal-Ligand Bonds Than Their Neutral Counterparts

“…At 6 K, a recorded axial signal additionally featured hyperfine coupling to the phosphine ligands ( Figure S3). The signal was simulated with effective g values g t = 2.338 and g k = 1.971, hyperfine coupling A t = 790 MHz (24.1 mT) to the 105 Pd metal center, and hyperfine coupling A k = 80 MHz (2.90 mT) to the two equivalent 31 P ligands. This finding demonstrates the delocalization of the unpaired spin on the phosphine ligands as predicted by DFT.…”

“…This finding demonstrates the delocalization of the unpaired spin on the phosphine ligands as predicted by DFT. The EPR spectrum of 11 recorded at room temperature showed an isotropic signal that was simulated with effective g value g iso = 2.172 and hyperfine coupling A iso = 200 MHz (6.58 mT) to the 105 Pd metal center ( Figure S4). The observed hyperfine coupling was significantly smaller than that of 3, indicating a higher amount of delocalization of unpaired spin from the metal center onto the ligands in 11.…”

Addition of Isocyanides to Tetramesityldigermene: A Comparison of the Reactivity between Surface and Molecular Digermenes

“…Only a few examples of the CN stretching vibration in isocyanides coordinated to germanium have been published: 2092 cm −1 for 13 , 2113 cm −1 for 14 , [ 15] 2132 cm −1 for 15 (R= t Bu), and 2165 cm −1 for 15 in which R=Me; however, it is difficult to make a meaningful comparison between the CN stretching frequencies of these complexes and that of the surface adduct. Tuononen has published a detailed computational analysis of the orbital interactions in germylene–isocyanide complexes and the influence of complexation on the characteristic stretching frequency of the isocyano functional group . In the absence of any π‐type back‐bonding, an increase in the wavenumber compared to the ν (CN) of the uncomplexed isocyanide is noted.…”

“…Tuononen has published adetailed computationala nalysis of the orbital interactions in germylene-isocyanide complexes and the influence of complexation on the characteristic stretching frequency of the isocyano functional group. [22] In the absence of any p-type back-bonding, an increase in the wavenumber compared to the n(CN) of the uncomplexed isocyanide is noted. With al one pair on germani-um, p-back donation can occur;h owever,t he orbital interaction must be quite significant before ad ecrease in the wavenumber of n(CN) is observed relative to the uncomplexed isocyanide.…”