# Homoleptic seven-coordinate Ti(0) and Zr(0) through a new stabilization mode

## TL;DR

Scientists created new titanium and zirconium complexes with zero oxidation states using gallium ligands, revealing unique bonding interactions.

## Contribution

The first homoleptic seven-coordinate Ti0 and Zr0 complexes stabilized by GaTMP ligands are synthesized and analyzed.

## Key findings

- The complexes [Ti(GaTMP)7] and [Zr(GaTMP)7] are stable and exclusively coordinated by metalloligands.
- Ga⋯Ga covalent interactions, enabled by π-backbonding, are crucial for stabilizing the complexes.
- Oxidation of [Zr(GaTMP)7] forms [Zr(GaTMP)8]2+ with similar bonding features.

## Abstract

The stabilization of zero oxidation state group (IV) metal centers in readily accessible compounds on a preparative scale remains a significant challenge. Substituting hydrocarbon ligands in the precursor complexes [Ti(η6-toluene)2] and [Zr(η6-cycloheptatriene)2] with monovalent GaTMP (TMP = 2,2,6,6-tetramethylpiperidinyl) yields the first homoleptic seven-coordinate Ti0 and Zr0 complexes, [Ti(GaTMP)7] (1) and [Zr(GaTMP)7] (2), which are exclusively coordinated by metalloligands. The bonding situation of 1 and 2 was rationalized through DFT calculations, revealing the critical importance of tangential Ga⋯Ga covalent interactions for stabilizing the compounds. Oxidation of 2 leads to the formation of [Zr(GaTMP)8]2+ (5), showcasing a similar bonding situation. These Ga⋯Ga interactions arise from significant π-backbonding from the Ti0 and Zr0 centers into the constructive combinations of the diffuse 4p orbitals of the Ga(i) centers. This unique cooperative feature of the all-Ga metalloligand sphere marks a clear distinction from the bonding properties of formally isolobal carbonyls, phosphines, or N-heterocyclic carbenes.

The first homoleptic Ti0 and Zr0 complexes stabilized by gallylene ligands reveal unprecedented bonding: Ga–Ga interactions enabled by π-backbonding into diffuse Ga 4p orbitals. This unique ligand sphere defies classical carbonyl analogies.