Metal-ligand bond in group-11 complexes and nanoclusters

Abstract

Density functional theory is used to study geometric, energetic, and electronic properties of metal-ligand bonds in a series of group-11 metal complexes and ligand-protected metal clusters. We study complexes as the forms of M-L (L = SCH$_3$, SC$_8$H$_9$, PPh$_3$, NHC$^M{}^e$, NHC$^E{}^t$, NHC$^i{}^P{}^r$, NHC$^B{}^n$, CCMe, CCPh) and L$_{1}$-M-L$_{2}$ (L$_{1}$ = NHC$^B{}^n$, PPh$_3$, and L$_{2}$ = CCPh). Furthermore, we study clusters denoted as [M$_1{}_3$L$_6$Br$_6$]$^-$ (L = PPh$_3$, NHC$^M{}^e$, NHC$^E{}^t$, NHC$^i{}^P{}^r$, NHC$^B{}^n$). The systems were studied at the standard GGA level using the PBE functional and including vdW corrections via BEEF-vdW. Generally, Au has the highest binding energies, followed by Cu and Ag. PBE and BEEF-vdW functionals show the order Ag-L > Au-L > Cu-L for bond lengths in both M-L complexes and metal clusters. In clusters, the smallest side group (CH$_3$) in NHCs leads to the largest formation energy whereas no significant variations are seen concerning different side groups of NHC in M-L complexes. By analyzing the projected density of states and molecular orbitals in complexes and clusters, the M-thiolate bonds were shown to have $\sigma$ and $\pi$ bond characteristics whereas phosphines and carbenes were creating $\sigma$ bonds to the transition metals. Interestingly, this analysis revealed divergent behavior for M-alkynyl complexes: while the CCMe group displayed both $\sigma$ and $\pi$ bonding features, the CCPh ligand was found to possess only $\sigma$ bond properties in direct head-to-head binding configuration. Moreover, synergetic effects increase the average binding strength to the metal atom significantly in complexes of two different ligands and underline the potential of adding Cu to synthesize structurally richer cluster systems. This study helps in understanding the effects of different ligands on the stability of M-L complexes and clusters and suggests that PPh$_3$ and NHCs-protected Cu clusters are most stable after Au clusters.