Static magnetic susceptibility measurements as well as ESR measurements at 9 GHz on two complex M⁺(TCNQ)₂^- salts are reported. The results reveal that the on-site interaction between electrons is strongly reduced for its 'bare ion' value, though the on-site interaction is still larger than the total one-electron bandwidth. At high temperatures (20-250K) the electronic structure of both materials shows (pseudo one-dimensional) correlated metal features (finite density of states at Fermi energy, enhanced moments). At low T ( approximately 20K) the susceptibility measurements indicate a rather sudden breakdown of the correlated metal behaviour. Arguments are given that the short-range antiferromagnetic interaction must be held responsible for this breakdown. Below the 'transition', one of the materials indeed shows the spin behaviour of an antiferromagnetic Heisenberg insulating state. The low-T spin behaviour of the other salt is rather obscured by an additional contribution to the spin susceptibility, presumably due to impurities.