  Molecular electronic spins represent one of the most promising building
blocks for the design of quantum computing architectures. However, the
advancement of this technology requires the increase of spin lifetime at
ambient temperature. Spin-phonon coupling has been recognized as the key
interaction dictating spin relaxation at high temperature in molecular crystals
and the search for chemical-design principles to control such interaction are a
fundamental challenge in the field. Here we present a multi-reference
first-principles analysis of the g-tensor and the spin-phonon coupling in a
series of four exa-coordinate Vanadium(IV) molecular complexes, where the
catecholate ligand donor atom is progressively changed from Oxygen to Sulphur,
Selenium and Tellurium. A ligand field interpretation of the multi-reference
electronic structure theory results made it possible to rationalize the
correlation between the molecular g-shifts and the average spin-phonon coupling
coefficients, revealing the role of spin-orbit coupling, chemical bond
covalency and energy splitting of d-like orbitals in spin relaxation. Our study
reveals the simultaneous increase of metal-ligand covalency and electronic
excited state energy separation as key elements of an optimal strategy towards
long spin-lattice lifetimes in molecular qubits.
