Metal complexes as ligands. IV. Structures of bis[bis(triphenylphosphine)silver(I)]bis[1,2-dicyano-1,2-ethylenedithiolato)nickelate(II), and of the bis[bis(triphenylphosphine)silver(I)] bis(1,1-dicyano-2,2-ethylenedithiolato)nickelate(II)