Pd and Ni dimers supported by PSiP ligands in which two hypervalent five-coordinate Si atoms bridge the two metal centers are reported. Crystallographic characterization revealed a rare square-pyramidal geometry at Si and an unusual asymmetric M2Si2 core (M=Pd or Ni). DFT calculations showed that the unusual structure of the core is also found in a model in which the phosphine and Si centers are not part of a pincer group, thus indicating that the observed geometry is not imposed by the PSiP ligand. NBO analysis showed that an asymmetric four-center two-electron (4c-2e) bond stabilizes the hypervalent Si atoms in the M2Si2 core.