Solid state multinuclear NMR study of σ-acetylide complexes of platinum, trans-[ClPt(P<sup>n</sup>Bu<sub>3</sub>)<sub>2</sub>CCpC<sub>6</sub>H<sub>4</sub>CCPt(P<sup>n</sup>Bu<sub>3</sub>)<sub>2</sub>Cl] and trans-[Pt(P<sup>n</sup>Bu<sub>3</sub>)<sub>2</sub>CCpC<sub>6</sub>H<sub>4</sub>CC]<sub>n</sub>