see, if you are not satisfied by above 2 answers or you want stats:
Look at the electron affinity.
long answer: if metal has to form a bond then essential criteria of the bond formation is that electron "revolve" around 2 atoms (also spend most time in the space between 2 nucleus) and for it to revolve around, it needs to bind strongly, one metal atom has to accomodate it. To accomodate an electron in a metal, one needs to put 45-60 Kj/mol of the energy (for comparison energy available at room temp is somewhat around 2 Kj/mol).
