Mercury(II) halide complexes of tertiary phosphines. Part X. Study of the triheteroarylphosphine complexes HgX2(PR3)2 [X = Cl, Br, I;R = 2-furyl or 2-thienyl]. Crystal structure of HgCl2[P(2-thienyl)3]2. Solid state vibrational and 31P NMR solution studies