Group
11 dihydrides MH
2
–
(M = Cu,
Ag, Au, Rg) have been much less studied than the corresponding MH
compounds, despite
having potentially several interesting applications in chemical research. In this work, their
main spectroscopic constants (bond lengths, dissociation energies,
and force constants) have been evaluated by means of highly accurate
relativistic four-component coupled cluster (4c-CCSD(T)) calculations
in combination with large basis sets. Periodic trends have been quantitatively
explained by the charge-displacement/natural orbitals for chemical
valence (CD-NOCV) analysis based on the four-component relativistic
Dirac–Kohn–Sham method, which allows a consistent picture
of the nature of the M–H bond to be obtained on going down
the periodic table in terms of Dewar–Chatt–Duncanson
bonding components. A strong ligand-to-metal donation drives the M–H
bond and it is responsible for the heterolytic (HM···H
–
) dissociation energies to increase monotonically from
Cu to Rg, with RgH
2
–
showing the strongest
and most covalent M–H bond. The “V”-shaped trend
observed for the bond lengths, dissociation energies, and stretching
frequencies can be explained in terms of relativistic effects and,
in particular, of the relativistically enhanced sd hybridization occurring
at the metal, which affects the metal–ligand distances in heavy
transition-metal complexes. The
sd
hybridization
is very small for Cu and Ag, whereas it becomes increasingly important
for Au and Rg, being responsible for the increasing covalent character
of the bond, the sizable contraction of the Au–H and Rg–H
bonds, and the observed trend. This work rationalizes the spectroscopic/bond
property relationship in group 11 dihydrides within highly accurate
relativistic quantum chemistry methods, paving the way for their applications
in chemical bond investigations involving heavy and superheavy elements.