The co-ordination of small molecules by manganese(II) phosphine complexes. Part 14. Some novel mixed tertiary phosphine–t-butyl isocyanide complexes of manganese(II): [MnBr2(CNBut)(PPhMe2)] and [Mn(CNBut)4(PR3)2]2+(PR3= PPhMe2, PPh2Me, PPhEt2, or PPrn3), and the X-ray crystal structure of bis(dimeth