Diphosphine bifunctional chelators for low-valent metal ions. Crystal structures of the copper(I) complexes [CuClL<sup>1</sup><inf>2</inf>] and [CuL<sup>1</sup><inf>2</inf>][PF<inf>6</inf>] [L<sup>1</sup> = 2,3-bis(diphenylphosphino)maleic anhydride]