UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg} 2 ] 3 (Priso = [(DipN) 2 CNPr i 2 ] À , Dip = 2,6-diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic CÀ H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg} 2 (μ-H)(μ-Ar)] 4-9. In contrast to similar reactions reported for β-diketiminate coordinated counterparts of 3, these CÀ H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p-NC 5 H 4 Bu t (py But ), gave [(Priso)Mg(py But H)(py But ) 2 ] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(py But* )(py But ) 2 ] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene CÀ H activations. One of these involves photochemical cleavage of the MgÀ Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg * . These then doubly reduce the arene substrate to give "Birch-like" products, which subsequently rearrange via CÀ H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic CÀ H activation process and formation of an alkylbridged magnesium(II) species, [{Mg(μ-Priso À H )} 2 ] 12. Furthermore, irradiation of a 1 : 1 mixture of 3 and the β-diketiminato dimagnesium(I) compound, [{( Dip Nacnac)Mg} 2 ] ( Dip Nacnac = [HC(MeCNDip) 2 ] À ), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)MgÀ Mg( Dip Nacnac)] 13. Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species.

Reactions of a dimagnesium(I) compound, [{(DipNacnac)Mg}2] (DipNacnac = [HC(MeCNDip)2]-, Dip = 2,6-diisopropylphenyl), pre-activated by coordination with simple Lewis bases (4-dimethylaminopyridine, DMAP; or TMC, :C(MeNCMe)2), with 1 atmosphere of CO in the presence of one equivalent of Mo(CO)6 at room temperature, led to the reductive tetramerisation of the diatomic molecule. When the reactions were carried out at room temperature, there is an apparent competition between the formation of magnesium squarate, [{(DipNacnac)Mg}(C4O4){-Mg(DipNacnac)}]2, and magnesium metallo-ketene products, [{(DipNacnac)Mg}{-(C4O4)Mo(CO)5}{Mg(D)(DipNacnac)}], which are not inter-convertible. Repeating the reactions at 80 °C led to the selective formation of the magnesium squarate, implying that this is the thermodynamic product. In an analogous reaction, in which THF is the Lewis base, only the metallo-ketene complex, [{(DipNacnac)Mg}{-(C4O4)Mo(CO)5}{Mg(THF)(DipNacnac)}] is formed at room temperature, while a complex product mixture is obtained at elevated temperature. In contrast, treatment of a 1:1 mixture of the guanidinato magnesium(I) complex, [(Priso)Mg‒Mg(Priso)] (Priso = [Pri2NC(NDip)2]-), and Mo(CO)6, with CO gas in a benzene/THF solution, gave a low yield of the squarate complex, [{(Priso)(THF)Mg}(C4O4){-Mg(THF)(Priso)}]2, at 80 °C. Computational analyses of the electronic structure of squarate and metallo-ketene product types corroborate the bonding proposed, from experimental data, for the C4O4 fragments of these systems.