Two σ- and two π-dative quadruple bonds between the s-block element and transition metal in [BeM(CO) 4 ; M = Fe - Os].

We report the chemical bonding and reactivity of the first example of neutral 18 valence electron transition metal complexes of beryllium, [BeM(CO) 4 ; M = Fe - Os], in trigonal bipyramidal coordination geometry, where the bonding between the transition metal and the s-block element beryllium (M-Be) can be best described by dative quadruple bonds. In contrast to the conventional multiple bonding pattern, the quadruple bonds comprise two σ-bonds and two π-bonds, viz. , one Be → M σ-bond, one M → Be σ-bond, and two M → Be π-bonds. Since the M-Be quadruple bonds are described by dative interactions, the Be centre shows ambiphilic character as indicated by the high proton and hydride affinity values.