Re2(CO)6(μ-thpymS)2 (thpymSH = pyrimidine-2-thiol) as a versatile precursor to mono- and polynuclear complexes: X-ray crystal structures of fac-Re(CO)3(PPh3)(κ2-thpymS) and two isomers of ReRu3(CO)13(μ3-thpymS)